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THE  LIBRARY 

OF 

THE  UNIVERSITY 
OF  CALIFORNIA 

LOS  ANGELES 


•.'V 


if!  V, 


REFRACTION 

INCLUDING 

MUSCLE  IMBALANCE  AND  THE  ADJUSTMENT 
OF  GLASSES 


ROYAL  S.  COPELAND,  A.  M.,  M.  D., 

PROFESSOR    IN    THE    UNIVERSITY   OF   MICHIGAN 


ADOLPH  E.  IBERSHOFF,  M.  D., 

INSTRUCTOR   IN   THE   UNIVERSITY  OF   MICHIGAN 


PHILADELPHIA  : 

BOERICKE  &  TAFEL 
1906 


COPYRIGHTED 
BY 

BOERICKE  &  TAFKL 
1906 


PREFACE. 


With  no  word  of  explanation,  it  would  be  presumptuous 
to  add  to  the  large  and  growing  list  of  excellent  and  ex- 
haustive treatises  on  the  subject  of  refraction.  No  student 
of  medicine,  we  think,  will  deny  that  the  study  of  this 
branch  of  practical  therapeutics  is  beset  with  many  difficul- 
ties, and,  when  viewed  through  the  meshes  of  a  multiplicity 
of  diagrammatic  cobwebs,  it  appears  especially  unattract- 
ive. We  feel  safe  in  adding  that  any  real  help  in  simplifying 
the  teaching  of  refraction  will  be  welcomed  by  student  and 
practitioner.  An  attempt  to  accomplish  this  end  is  our  only 
hope  of  justification,  and  if,  in  a  small  measure,  we  succeed 
in  elucidating  some  of  the  difficult  principles  and  problems 
of  the  subject,  we  shall  feel  that  our  efforts  have  not  been 
vainly  spent. 

Refraction  is  an  eminently  practical  science  and  we  aim 
to  treat  it  as  such,  divesting  it,  so  far  as  possible,  of  such 
theoretical  demonstrations,  logarithmic  computations,  and 
minor  technicalities  as  are  not  deemed  absolutely  essential 
to  a  thorough  comprehension  of  the  subject.  To  strip  it  of 
its  embellishments  is  to  invite  criticism,  but  the  relative  im- 
portance of  what  has  been  omitted  is,  of  course,  a  matter  of 
judgment.  To  some  of  our  readers,  occasional  statements 
may  seem  too  broad,  but  if  we  have  failed  to  limit  them  suf- 
ficiently, -or  to  state  all  known  exceptions,  we  have  acted  in 
the  hope  of  avoiding  confusion  without  sacrifice  of  essentials. 
To  lead  the  student  along  practical  lines  to  an  accurate  un- 


IV  PREFACE. 

derstanding  of  the  applied  principles  of  refraction  has  been 
the  object  of  our  labor.  To  this  end,  it  is  strongly  recom- 
mended that  the  student  early  acquaint  himself  with  the 
test-case  and,  so  far  as  possible,  supplement  his  studies  by 
practical  verification  of  the  facts  and  theories  set  forth  in 
the  text. 

University  of  Michigan,  June,  1905. 


CONTENTS. 


CHAPTER  I. 

PAGE. 

OPTICAL  PRINCIPLES 9 

Wave  Theory  of  Light.  9  ;  The  Light  Ray,  9  ;  Refraction,  10 ;  Index  of 
Refraction,  11;  First  Principles,  11. 

CHAPTER   II. 

LENSES, 13 

A  Lens, 13;  The  Prism, 13;  Spherical  Lenses,  14;  Cylindric  Lenses,  20; 
Properties  of  Lenses,  23 ;  Varieties  of  Lenses,  23 ;  Systems  of  Lens 
Measurements,  24;  The  English  System,  25;  The  Metric  System, 
25 ;  Prescription  for  Lenses,  26 ;  Neutralization  of  Lenses,  26 ; 
Other  Methods,  28;  Measuring  the  Strength  of  a  Prism,  29;  Me- 
chanical Devices  for  Measuring  Lenses,  31 ;  The  Relationship  of 
Lenses,  32;  The  Relationship  Between  Spheric  and  Cylindric 
Lenses,  32;  The  Relationship  of  Cylinders,  33;  The  Relationship  of 
Spheres  to  Prisms,  34;  The  Relationship  of  Cylinders  to  Prisms,  34. 

CHAPTER   III. 

THE  NORMAL  EYE, 35 

Introduction,  35;  Requisites  of  Vision,  35  ;  Dioptric  System  of  the  Eye, 
35  ;  Static  Refraction,  36  ;  Dynamic  Refraction,  36  ;  Physiology  of 
Accommodation,  37 ;  Convergence,  37;  Relationship  of  Accommo- 
dation, Convergence  and  Pupillary  Contraction  in  the  Emmetropic 
Eye,  40;  Optical  Defects  of  the  Normal  Eye,  41 ;  Changes  Occur- 
ring During  the  Life  History  of  the  Eye,  42 ;  Visual  Acuity,  44. 

CHAPTER   IV. 

ASTHENOPIA, " 47 

Symptoms,  47 ;   Asthenopia  as  a  Cause  of  General  Disease,  48. 

CHAPTER  V. 

THE  TRIAL  CASE  AND  ITS  USES, ...    49 

Introduction,  49  ;  Spherical  Lenses,  49;  Cylindric  Lenses,  49;  Prismatic 
Lenses,  50;  The  Trial  Frame,  52. 


VI  CONTENTS. 

0 

CHAPTER   VI. 

HYPEROPIA  OR  FAR  SIGHT, 55 

Introduction,  55;  Causes,  57;  Symptoms  and  Sequela;,  57;  Spasm  of 
Accommodation,  57;  Varieties  of  Hyperopia,  58;  Determination 
of  Hyperopia,  58 ;  Correction  of  Hyperopia,  58;  Cycloplegics  and 
Mydriatics,  60 ;  Contraindication  for  the  Use  of  Mydriatics,  60 ; 
Correction  of  Results  Obtained  Under  a  Cycloplegic,  61;  Myotics,  62. 


CHAPTER   VII. 

MYOPIA  OR  NEAR  SIGHT, 63 

Introduction,  63 ;  Causes  of  Myopia,  65  ;  Symptoms  of  Myopia,  65 ; 
Determination  of  Myopia,  66 ;  Correction  of  Myopia,  66  ;  Pro- 
phylaxis, 67. 

CHAPTER   VIII. 

ASTIGMIA, 68 

Introduction,  68;  Forms  of  Regular  Astigmia,  69;  Causes  of  Regular 
Astigmia,  71;  Symptoms,  71;  Determination  of  Astigmia,  71; 
Correction  of  Astigmia,  74 ;  Simple  Hyperopic  Astigmia,  75 ; 
Simple  Myopic  Astigmia,  76 ;  Compound  Hyperopic  Astigmia,  77; 
Compound  Myopic  Astigmia,  78  ;  Mixed  Astigmia,  79. 


CHAPTER   IX. 

ANISOMETROPIA,  APHAKIA,  AMBLYOPIA  AND  MALINGERING, 82 

Anisometropia,  82;  Aphakia,  82;  Amblyopia  and  Amaurosis,  82  ;  Ala- 
lingering,  83. 

CHAPTER   X. 

PRESBYOPIA, 84 

Causes  and  Symptoms,  84;  The  Relationship  Between  Presbyopia  and 
the  Age  of  the  Patient,  86 ;  Correction  of  Presbyopia,  86. 


CHAPTER   XI. 

CONFIRMATORY  TESTS  (OBJECTIVE), .    .    88 

Introduction,  88;   Mirror  Test  (Direct  Method),  89;    Mirror  Test  (In- 
direct Method),  90;  Retinoscopy  or  Skiascopy  (Keratoscopy),  91. 


CONTENTS.  Vll 

CHAPTER   XII. 

MI/SCULAR   IMBALANCE 94 

Ocular  Movements,  94;  Muscular  Balance  or  Orthophoria,  95  ;  Hetero- 
tropia  or  Strabismus,  95 ;  Heterophoria,  96 ;  Forms  of  Hetero- 
phoria,  96;  Determination  of  Heterophoria,  96  ;  Prism  Test,  98; 
Maddox  Test,  98 ;  Cobalt  Test,  99  ;  Cover  Test,  99  ;  Determina- 
tion of  Cyclophoria,  100 ;  Correction  of  Heterophoria,  100 ;  Cor- 
rection of  the  Error  of  Refraction,  100  ;  Prism  Exercise,  101;  Wear- 
ing of  Prisms,  101 ;  Operation,  102. 


CHAPTER   XIII. 

SPECTACLES  AND  EYE  GLASSES, 103 

Introduction,  103 ;  Spectacles,  103 ;  Requisites  of  Properly  Fitting 
Glasses,  105 ;  Measurements  and  Data  for  Ordering  Spectacles, 
105 ;  Inter-pupillary  Distance,  106 ;  Size  of  Lenses,  107 ;  Angle 
of  Lenses,  107  ;  Bridge  or  Nose  Piece,  107 ;  Temples,  108 ;  Eye 
Glasses,  108 ;  Measurements  and  Data  for  Ordering  Eye  Glasses. 
109;  Springs.  109;  Studs,  110;  Guards,  110;  Other  Forms  of 
Glasses,  111;  Half  Oval  Eye  Frames,  111 ;  Reversible  Spectacles, 
111;  Hook  or  Grab  Fronts,  111;  Clerical  or  Adjustable  Eye 
Glasses,  112. 

CHAPTER    XIV. 

SPECIAL  FORMS  OF  LENSES, 113 

Bifocal  Lenses,  113;  Periscopic  Lenses,  114;  Meniscus  Lens,  115;  Toric 
Lens,  115. 

APPENDIX. 

MECHANICAL  AIDS  TO  REFRACTION, 118 

Ophthalmoscope,  119;  Luminous  Ophthalmoscope,  119;  Luminous 
Retinoscope,  120;  Ophthalmometroscope,  122;  Ametropometer, 
125;  Refractometer,  125;  Placido's  Disc,  126;  Ophthalmometer, 
127;  Risley's  Rotating  Prism,  129;  Phorometer,  129;  Perimeter, 
130;  Stigma  tome  ter,  132. 


INDEX  OF   ILLUSTRATIONS. 


FIGURE.  PAGE. 

1.  The  Refraction  of  Light, n 

2.  A  Prismatic  or  Wedge-shaped  Lens, .'   .    .    .  13 

3.  The  Deflection  of  a  Light  Ray  Toward  the  Base  of  a  Prism,    ....    14 

4.  Sections  of  a  Sphere  to  Represent  Convex  Spherical  Lenses,     ...    14 

5.  A  Concave  Spherical  Lens, 15 

6.  The  Convergence  of  a  Beam  of  Light  in  Passing  Through  a  Spherical 

Convex  Lens, .     15 

7.  The  Divergence  of  a  Beam  of  Light  in  Passing  Through  a  Concave 

Lens, .    .  16 

8.  The  Virtual  Focus  of  a  Concave  Lens,    ....  16 

9.  Sections  of  Spherical  Lenses  as  Made  Up  of  Two  Prisms,     ...        17 

10.  Spherical  Convex  Surfaces  as  Made  Up  of  Prism  Sides, 18 

n.  Spherical  Concave  Surfaces  as  Made  Up  of  Prism  Sides, 18 

12.  Method  of  Determining  the  Optical  Centre  of  a  Lens, 19 

13.  Convex  Cylindrical  Lens,    .    .  20 

14.  How    a    Beam    of    Light    Becomes   Wedge-shaped    After    Passing 

Through  a  Cylinder,  .  .  21 

15.  A  Concave  Cylindrical  Lens, 22 

16.  How  the  Axis  of  a  Cylindrical  Lens  is  Indicated, 23 

17.  Varieties  of  Lenses 24 

18.  Break  in  the  Continuity  of  a  Line  Caused  by  Tilting  a  Cylindrical 

Lens, 2y 

19.  Break  in  the  Continuity  of  a  Line  Caused  by  a  Prismatic  Lens,  .    .    30 

20.  Authors'  Axis  Finder,  . 31 

21.  Lens  Measure, 31 

22.  Prismatic  Effect  of  Decentering  a  Lens, 34 

23.  Mechanism  of  Accommodation, 38 

24.  Method  of  Constructing  Snellen's  Test  Letters,  .  .    .    44 

25.  Principle  Involved  in    Snellen's  Test  Letters, -44 

26.  Test  Card, 45 

27.  Changeable  Test  Card, 46 

28.  Trial  Case,     .    .  49 

29.  Cylindrical  Test  Lens, 50 

30.  Pin  Hole  Disc, .'    .    .    .  51 

31.  Stenopaeic  Disc, .....    51 

32.  Maddox  Multiple  Rod, ...    51 


X  INDEX   OF   ILLUSTRATIONS. 

FIGURE.  PAGE. 

33.  Double  Prism, 51 

34.  Trial  Frame, 52 

35.  Trial  Frame  Properly  Adjusted  for  Conducting  a  Test,  .  53 

36.  Authors'  Trial  Frame  Tilted  for  Reading,  ....  54 

37.  The  Focus  of  Rays  in  a  Hyperopic  Eye,    .    . 

38.  A  Hyperopic  Eye  Corrected  for  Distant  Vision,  .    .  56 

39.  A  Hyperopic  Eye  Corrected  for  Near  Vision, .    .    56 

40.  The  Focus  of  Parallel  Rays  in  a  Myopic  Eye,  ....  .63 

41.  Why  a  Myopic  Eye  Sees  Clearly  at  Close  Range,      .    .  64 

42.  A  Myopic  Eye  Corrected  for  Distant  Vision,    ....  .    .    66 

43.  The  Effect  of  a  Double  Cylinderas  Illustrating  the  Refractive  Effect 

of  an  Astigmic  Surface, 68 

44.  Varieties  of  Astigmia, .    .    70 

45.  Fan  of  Rays  Used  in  Testing  for  Astigmia, 72 

46.  Astigmic  Dial, 73 

47.  Dial  as  Seen  by  an  Astigmic  Eye, 74 

48.  Position  of  Focal  Lines  in  Simple  Hyperopic  Astigmia  and  Position 

of  Correcting  Cylindric  Lens, 75 

49.  Position  of  Focal  Lines  in  Simple  Myopic  Astigmia  and  Position  of 

Correcting  Cylindric  Lens, 76 

50.  Position  of  Focal  Lines  in  Compound  Hyperopic  Astigmia  and  Posi- 

tion of  Correcting  Cylindric  Lenses, •    •    77 

51.  Position  of  Focal  Lines  in  Compound  Myopic  Astigmia  and  Position 

of  Correcting  Cylindric  Lenses, 78 

52.  Position  of  Focal  Lines  in  Mixed  Astigmia  and  Position  of  Correct- 

ing Cylindric  Lenses,  80 

53.  Gradual  Recession  of  the   Near   Point  Owing   to   Hardening  and 

Flattening  of  the  Lens — Presbyopia,  ...  85 

54.  Argand  Burner  with  Thorington  Retinoscopy  Chimney,  ......    88 

55.  Retinoscope,        89 

56.  Apparent  Displacement  of  an  Object  Toward  the  Apex  of  a  Prism,  .    97 

57.  Full  Frame  Spectacle  Mounting, ...  104 

58.  Rimless  or  Skeleton  Spectacle  Mounting, 104 

59.  Method  of  Measuring  Interpupillary  Distance, 106 

60.  Standard  Lens  Sizes, 107 

61.  Rimless  or  Skeleton  Eye  Glass  Mounting, 108 

62.  Full  Frame  Eye  Glass  Mounting,  109 

63.  Half  Oval  Eye  Glass  Frame, .    .    .  in 

64.  Grooved  Bifocal  Lens,  113 

65.  Different  Forms  of  Bifocal  Lenses,  .    .  114 

66.  A  Tor  with  Sections  to  Represent  Piano -Con vex  and  Piano-Concave 

Toric  Lenses, .    .    .    .  116 

67.  A  Toric  Lens, 116 

68.  Rear  View  of  Ophthalmoscope, 118 

69.  Luminous  Ophthalmoscope, 120 


INDEX   OF   ILLUSTRATIONS.  XI 

FIGURE.  PACK. 

70.  I/utninous  Retinoscope,  ...  121 

71.  Oplithalmonietroscope,.  .    .  122 

72.  Ametroporueter, '.    .    .  124 

73.  Refractometer,    .    . 125 

74.  Placido's  Disc,        ... 127 

75.  Ophthalmometer, 128 

76.  Risley's  Rotating  Prism, 129 

77.  Improved  Phorometer, 130 

78.  Perimeter,        .    .  .  131 

79.  Stigmatometer,       ....  132 


INDEX  OF  TABLES. 


TABLE.  PAGE 

I. — Showing  Strength  of  L,enses  and  Their -Focal  Length  in  English 

Inches  and  in  Centimeters, 26 

II. — Showing  the  Relationship  of  Accommodation,  Convergence  and 

Pupillary    Contraction, 41 

III. — Showing  Strength  of  Cycloplegics  and  Mydriatics,  Time  Required 

to  Act,  and  Duration  of  Drug  Effect, 61 

IV. — Showing  the  Relationship  Between  Presbyopia  and  the  Age  of  the 

Patient, 86 

V. — Showing  Ocular  Muscles  Concerned  in  the  Several  Excursions  of 

the  Eyeball,        94 

VI. — Showing  Relative  Position  of  Streak  to  Flame  in  the  Diagnosis  of 

Heterophoria, 99 


REFRACTION. 


CHAPTER  I. 
OPTICAL  PRINCIPLES. 

WAVE  THEORY  OF  LIGHT. 

The  long- time  accepted  view  regarding  light  production  is 
based  on  the  theory  that  all  space  is  pervaded  by  a  medium 
called  the  luminiferous  ether.  In  the  terms  of  this  hypothe- 
sis, the  emanations  from  a  luminous  source  involve  the  ether 
in  \vave-like  motion,  resulting  in  the  phenomenon  of  light. 

This  theory  answers  very  well  most  of  the  observed  phe- 
nomena, but  recently  it  has  been  considerably  revised.  The 
newer  view  is  that  the  oscillations  are  of  an  electrical 
nature,  that  electro-magnetic  radiations  and  light  radiations 
are  practically  identical.  The  nature  of  the  so-called  "elec- 
trical displacement"  is  not  thoroughly  understood,  but  in 
its  terms  all  of  the  phenomena  of  light  can  be  explained. 

It  is  believed  that  the  particles  constituting  the  ether  are 
in  constant  motion.  These  ethereal  waves,  according  to 
their  length,  frequency,  form  and  energy,  manifest  themselves 
by  stimulation  of  the  organs  of  special  sense.  In  one  in- 
stance, their  impingement  upon  the  terminal  nerves  of  the 
body  is  proved  by  a  sensation  of  feeling.  Again,  they  mani- 
fest themselves  as  heat  producers,  or  may  serve  to  induce 
chemical  changes.  Under  other  conditions  they  serve  to 
stimulate  the  retina  of  the  eye,  effecting  the  sensation  of 
light. 

THE   LIGHT   RAY. 

A  ray  of  light  may  be  considered  as  the  path  described  by 
the  emanation  from  a  single  point  in  the  luminous  source. 
2 


10  REFRACTION. 

Such  a  path  may  be  described  as  a  succession  of  waves  pro- 
ceeding along  a  straight  course.  This  has  been  compared  to 
the  oscillations  of  the  various  points  of  a  rope  along  which 
a  wave  is  passing,  the  vibration  of  the  ether  being  at  right 
angles  to  the  direction  of  the  wave  itself.  For  the  sake  of 
convenience  in  description,  these  oscillations  may  be  con- 
sidered as  consisting  of  "wave  fronts,"  or  "wave  crests." 
So  long  as  the  optical  density  of  the  transmitting  medium  is 
constant,  the  velocity  of  the  oscillations  is  uniform.  As 
would  be  expected,  their  speed  is  lessened  in  passing  from  a 
rare  into  a  dense  medium,  as,  for  instance,  from  the  air  into 
glass  or  water.  Their  direction  remains  unchanged,  pro- 
vided they  enter  the  denser  medium  perpendicularly.  This 
is  explained  by  the  fact  that  the  entire  transverse  vibration, 
"  wave  front,"  or  "  wave  crest,"  whatever  it  may  be  called, 
enters  the  denser  medium  at  the  same  time  and,  hence,  en- 
counters the  retarding  influence  equally  at  every  point. 

REFRACTION. 

Should  a  ray  of  light  encounter  in  its  course  a  transpar- 
ent medium  of  greater  density  and  enter  it  at  any  angle  less 
than  90°  (i.  e.,  not  perpendicularly),  one  end  of  the  "wave 
front,"  or  "wave  crest,"  will  be  retarded,  while  the  other 
end  proceeds  at  the  original  velocity  until  it,  too,  enters  the 
denser  medium.  (Fig.  1.)  In  this  way  the  course  of  the  ray 
is  deflected  from  its  straight  path  and  is  spoken  of  as  a  Re- 
fracted Ray.  The  reverse  of  this  takes  place  when  the  ray, 
at  any  angle  less  than  90°,  emerges  from  the  denser  medium 
to  enter  one  more  rare.  The  end  of  the  "  wave  front"  first 
liberated  immediately  increases  its  velocity  and  consequently 
a  deflection  occurs  in  its  course.  When  the  entire  "wave 
front"  has  passed  out  into  the  new  medium,  it  again  pro- 
ceeds in  a  straight  line. 

The  ray  before  its  deflection  is  known  as  the  Incident 
Ray,  and  the  substance  causing  its  change  of  course  is 
called  the  Refracting  Medium.  A  ray  of  light  entering  such 
a  refracting  medium  perpendicularly,  as  has  been  said,  will 
not  be  deflected,  although  its  velocity  will  be  diminished ; 
neither  will  the  refracted  ray  alter  its  course  while  travers- 


OPTICAL   PRINCIPLES. 


11 


ing  the  refracting  medium.  It  is  only  in  passing  from  one 
medium  into  another  of  different  density  that  any  change  of 
the  ray's  course  is  possible,  and  this  deflection  takes  place  at 
the  surface  of  the  refracting  medium. 

Refraction,  then,  is  the  deflection  of  luminous  rays  in 
passing  obliquely  from  one  medium  into  another  of  different 
optical  density. 

The  Index  of  Refraction.— Any  transparent  substance  is 
a  refracting  medium.  Some  media  deflect  rays  of  light  more 


FIG.   i. — Showing  the  Refraction  of  a  Light  Wave. 

than  does  air,  the  adopted  standard.  This  relative  refractive 
power  of  a  substance,  as  compared  with  air,  is  expressed  in 
figures  and  is  known  as  the  index  of  refraction.  It  varies 
directly  with  the  optical  density  of  the  medium.  Air  having 
an  index  of  1  and  crown  glass  having  half  again  as  much 
refractive  power,  because  of  its  greater  density,  is  said  to 
have  a  refractive  index  of  1.50. 

First  Principles. — From  what  has  been  said  we  ma}'  de- 
duct the  following  principles: 


12  REFRACTION. 

1.  From  any  luminous  source,   light  rays  pass   out  in 
every  direction. 

2.  Light  rays  always  proceed  in  a  straight  course,  if  the 
medium  through  which  they  pass  be  homogeneous;   if  de- 
flected from  their  original  direction  by  passing  through  an 
optically   denser   medium,    they   resume  a   straight    course 
upon  release. 

3.  The  velocity  with  which  the  rays  proceed  varies  in- 
versely  with   the  optical  density  of  the  medium   through 
which  they  pass. 

4.  Light  rays  passing  perpendicularly  from  a  rare  into 
an  optically  dense  medium,  or  vice  versa,  do  not  alter  their 
direction. 

5.  In  passing  obliquely  from  one  medium  into  another  of 
different   optical  density,  rays  of  light  are  deflected  from 
their  course. 

6.  The  degree  of  such  deflection  is  determined  by  : 

(a)  The  angle  of  entrance  ; 

(b)  The  difference  in  optical  density  (refractive  in- 

dices) of  the  two  media. 


CHAPTER  II. 

LENSES. 

A  Lens. — A  lens,  named  from  its  resemblance  to  a  lentil, 
may  be  any  transparent  substance,  the  surfaces  of  which 
cause  parallel  light  rays,  passing  through  it,  to  be  diverted 
from  their  course.  The  effect  of  a  lens  to  change  the  direc- 
tion of  a  light  ray,  as  has  been  said  in  a  previous  chapter 
(see  p.  11),  is  known  as  its  power  of  refraction.  To  under- 
stand this  it  is  well  to  consider  the  simplest  form  of  a  lens, 
the  optical  unit,  viz.,  the  wedge-shaped  lens  commonly 
known  as  a  prism. 


FIG.  2. — Showing  a  Prismatic  or  Wedge-shaped  Lens. 

The  Prism. — The  prism  is  a  transparent  substance  whose 
plain  surfaces  are  not  parallel  to  each  other.  (Fig.  2.)  The 
thick  edge  of  such  a  lens  is  spoken  of  as  the  base ;  the  thin 
edge  towards  which  the  surfaces  converge  being  the  apex. 
Rays  of  light  passing  through  a  prism  will  be  deflected 
towards  its  base.  (Fig.  3.)  When  a  light  ray  strikes  the 
inclined  surface  of  the  prism  one  end  of  the  wave  crests  is 
held  back  while  the  other  proceeds  uninterruptedly,  until 
it,  too,  encounters  the  more  resistent  lens  surface.  The  ray 
proceeds  along  its  new  course  through  the  lens  until  the 


14 


REFRACTION. 


other  surface  is  reached.    One  end  of  the  wave  crests  emerg- 
ng  into  the  less  resistent  air  advances  faster  than  the  other, 
'until  the  whole  wave-front  has  emerged,  when  th'e  ray  pro- 
ceeds along  its  new  course. 


FlG.  3. — Showing  the  Deflection  of  a  Light  Ray  Toward  the 
Base  of  a  Prism. 

Spherical  Lenses. — When  the  surface  of  a  lens  represents 
a  section  of  a  sphere,  it  is  called  a  Spherical  Lens.  (Figs.  4 
and  5.)  Such  a  lens  deflects  light  rays  equally  in  all  meridians. 


FIG.  4. — Showing  Sections  of  a  Sphere  to  Represent 
Convex  Spherical  Lenses. 


LENSES.  15 

Spherical  lenses  may  be  Convex  or  Concave.   When  thick- 
est at  the  centre,  the  lens  is  convex,  commonly  called  "  plus," 


FIG.  5. — Showing  a  Spherical  Concave  Surface. 

and  designated  by  the  sign  + .  The  effect  upon  a  beam  of 
light  in  passing  through  a  convex  spherical  lens  is  to  cause 
it  to  become  a  converging  cone.  (Fig.  6.)  The  apex  of  the 


FIG.  6. — Showing  the  Convergence  of  a  Beam  of  I<ight  in  Passing 
Through  a  Spherical  Bi-Convex  L,ens. 

cone,   the  point  to  which  the  rays  centre,  is  known  as  the 
Focus  of  the  lens. 


16 


REFRACTION. 


When  thickest  at  its  periphery,  the  lens  is  concave,  com- 
monly called  a  "minus"  lens,  designated  by  the  sign  —  .  A 
beam  of  light  in  passing  through  a  concave  spherical  lens 


FIG.  7. — Showing  the  Divergence  of  a  Beam  of  Light  in 
Passing  Through  a  Spherical  Bi-Concave  Lens. 

becomes  shaped  like  a  diverging  cone  truncated  at  the  sur- 
face of  the  lens.  (Fig.  7.)  Projected  backward  through  the 
lens  the  boundary  lines  of  this  cone  reach  a  point  on  the  side 


FIG.  8. — Showing  the  Virtual  Focus  of  a  Concave  Lens. 

from  which  the  light  enters.     This  point  is  known   as  the 

"negative"  or  "  virtual  focus"  of  the  concave  lens.  (Fig.  8.) 

It  has  long  been  customary  to  consider  a  convex  lens  as 


LENSES. 


17 


made  up  of  prisms,  placed  base  to  base,  and  a  concave 
lens  as  made  up  of  prisms  placed  apex  to  apex,  (Fig.  9.) 
Their  effect  upon  rays  of  light  is  thus  readily  explained.  But 
to  be  more  exact,  the  two  surfaces  of  a  spherical  lens  should 
be  considered  as  made  up  of  the  sides  of  an  infinite  number 
of  prism  sections.  When  the  bases  of  these  prism  sections 
are  directed  toward  the  centre  of  the  lens,  it  is  convex;  when 
they  are  directed  toward  the  edge  of  the  lens,  it  is  concave. 
(Figs.  10  and  11.)  In  both  concave  and  convex  lenses,  the 


FIG.  9. — Showing  How  Spherical  Lenses  may  be  Considered 
as  made  up  of  Prisms. 

angle  formed  by  the  two  surfaces  of  the  prism  sections  in- 
creases from  the  centre  outward.  At  the  centre  the  two  sur- 
faces are  parallel  and,  therefore,  have  no  refractive  effect  at 
that  part  of  the  lens,  if  the  light  enters  perpendicularly.  At 
the  periphery  the  prismatic  angle  is  largest,  hence  the  refrac- 
tive effect  is  greatest. 

A  straight  line,  perpendicular  to  both  surfaces  of  the  lens 
and  passing  through  its  centre  of  curvature,  is  called  the 
Principal  Axis.  The  Focus,  whether  it  be  the  real  focus  of 
the  convex  lens,  or  the  virtual  focus  of  the  concave  lens,  is 


18 


REFRACTION. 


FIG.  10. — Showing  Convex  Spherical  Surfaces  as  made  up  of 
Prism  Sides. 


FIG.  ii. — Showing  Concave  Spherical  Surfaces  as  made  up  of 
Prism  Sides. 


LENSES. 


19 


found  in  the  Principal  Axis.    The  distance  of  the  focus  from 
the  lens  is  called  the  Focal  Distance. 

The  Optical  Centre  of  a  lens  is  that  point  on  the  principal 


FIG.   12. — Showing  Method  of  Determining  the  Optical 
Centre  of  a  L/ens. 


axis  and  within  the  substance  of  the  lens,  through  which  rays 
coming  from  all  directions  pass  without  altering  their 
course.  It  may  be  found  by  viewing  the  corner  of  a  test 


20  REFRACTION'. 

card  or  other  object  presenting  a  right  angle.  When  the  two 
lines  seen  through  the  lens  are  continuous  with  those  outside 
of  it,  the  point  of  the  angle  will  coincide  with  the  optical 
centre  of  the  lens.  (Fig.  12.) 

Cylindric  Lenses. — When  a  lens  diverts  light  rays  in  one 
plane  only,  it  is  called  a  Cylindric  or  Astigmic  Lens.  This 
is  made  by  cutting  lengthwise  through  a  solid  cylinder  or 


FIG.   13. — Showing  Convex  Cylindric  Lens. 


rod  of  glass,  splitting  off  a  shaving,  as  it  were.  (Fig.  13.) 
The  resulting  segment,  of  course,  will  have  one  plane  and 
one  cylindric surface, constitutingaplano-cylindric  lens.  The 
axis  of  such  a  lens  coincides  with  the  axis  of  the  rod  from 
which  it  is  cut;  in  other  words,  it  corresponds  to  the  lead  of 
the  lead-pencil,  were  that  the  cylinder  from  which  the  shav- 
ing came. 


LENSES. 


21 


A  cylindric  lens  causes  transmitted  rays  to  meet,  not  at  a 
focal  point,  as  happens  with  a  spherical  lens,  but  in  a  focal 
line.  A  beam  of  light,  after  passing  through  such  a  lens,  be- 
comes wedge-shaped.  (Fig.  14.)  The  sharp  edge  of  the 
wedge  is  the  focal  line  and  this  lies  parallel  with  the  axis  of 
the  lens.  The  plane  in  which  light  rays  are  deflected  by  a 


FIG.   14. — Showing  Effect  on  a  Beam  of  Light  Passing 
Through  a  Cylindric  Lens. 


cylindric  lens  is,  therefore,  at  right  angles  to  the  axis;  the 
rays  lying  in  a  plane  with  the  axis  remain  unchanged. 

Cylindric  lenses,  or  Cylinders,  as  they  are  called,  may  be 
convex  or  concaves  according  as  they  cause  transmitted 
rays  to  converge  or  diverge. 


22  REFRACTION. 

In  describing  a  cylindric  lens,  it  is  necessary  to  indicate 
its  axis  (the  non-refracting  meridian).     With  the  lens  in  its 


FIG.   15. — Showing  a  Concave  Cylindric  Lens. 

proper  position  before  the  eye,  the  axis  is  indicated  in  de- 
grees of  a  semi-circle  numbered   from  right  to  left.     If  the 


FIG.   16. — Showing  How  the  Axis  of  a  Cylindric  Lens  is  Indicated. 

axis  lies  horizontally,  it  is  denoted  as  180°,  if  vertically,  as 
90°,  etc.     (Fig.  16.) 


LENSES.  23 


PROPERTIES    OF    LENSES. 

Convex  Spherical  Lenses. — From  what  has  been  said  it 
will  be  seen  that : 

1.  Parallel  rays  passing  through  a  spherical  convex  lens 
are  made  to  converge  to  a  point,  or  focus.    The  distance  of 
this  point  from  the  centre  of  the  lens,  measured  along  the 
principal  axis,  is  called  the  focal  distance. 

Conversely,  rays  proceeding  from  the  principal  focus  of  a 
convex  lens  are  rendered  parallel  by  the  action  of  the  lens. 

2.  Divergent  rays,  after  traversing  such  a  lens,  unite  at  a 
point  farther  away  from  the  lens  than  the  principal  focus. 

Conversely,  rays  proceeding  from  a  point  farther  from 
the  lens  than  its  principal  focus  emerge  from  it  as  con- 
vergent rays. 

3.  Convergent  rays,  refracted  by  a  convex  lens,  meet  at  a 
point  nearer  to  the  lens  than  its  principal  focus. 

Conversely,  rays  emanating  from  this  point  are,  upon  re- 
fraction, made  less  divergent. 

Concave  Spherical  Lenses. — Parallel  rays  after  passing 
through  a  concave  lens  diverge  as  though  emitted  from  a 
point  on  the  side  from  which  the  light  emanates.  This  point 
is  the  "virtual"  focus  of  the  lens  and  its  distance  from  the 
lens  is  the  "  negative  "  focal  length. 

Rays  converging  to  meet  at  this  point  become  parallel 
after  passing  through  the  lens. 

Cylindrical  Lenses. — The  same  properties  characterizing 
spherical  lenses  apply  to  cylinders,  but  in  one  meridian  only, 
viz.,  at  right  angles  to  the  axis. 

VARIETIES    OF    LENSES. 

Every  lens  has  two  refracting  surfaces,  the  shape  and 
relative  position  of  these  surfaces  to  each  other  determining 
the  power  and  refracting  properties  of  the  lens.  (Fig.  17.) 
Both  surfaces  may  be  plane,  spherical,  or  cylindrical,  or  one 
surface  ma\T  differ  from  the  other;  therefore,  a  large  variety 
of  lenses  may  be  ground.  Those  in  common  use  are  the  fol- 
lowing : 


24 


REFRACTION. 


1.  Piano;  2.  Prismatic;  3.  Plano-convex  Spherical;  4. 
Plano-concave  Spherical;  5.  Plano-convex  Cylindrical;  6. 
Plano-concave  Cylindrical ;  7.  Biconvex  Spherical ;  8.  Bi- 
concave Spherical ;  9.  Concavo-convex  (two  varieties):  (a) 
The  Convex  surface  having  a  greater  radius  of  curvature 
than  the  concave,  forming  a  lens  called  the  Converging 
Meniscus,  (b)  The  Convex  surface  having  a  lesser  radius  of 
curvature,  forming  the  Diverging  Meniscus;  10.  Convex 
Sphero-cylindrical;  11.  Concave  Sphero-cylindrical ;  12.  Bi- 
convex Cylindrical ;  13.  Biconcave  Cylindrical ;  14.  Con- 
cavo-convex Cylindrical  (with  axes  crossed).  A  prism  may 


FIG.  17. — Showing  Varieties  of  Lenses,  viz.:   i,  Piano;  2,  Bi-Convex 

Spherical;  3,  Piano-Con  vex  Spherical;  4,  Bi-Concave  Spherical; 

5,  Piano-Concave  Spherical;   6,  Converging  Meniscus;  7, 

Diverging  Meniscus;  8,    Coquille;   9,    M'Coquille; 

10,  Prism;  n,  Convex  Cylindrical; 

12,  Concave  Cylindrical. 


be  introduced  as  an  element  in  each  of  the  above  named 
lenses,  thus  doubling  the  list  of  possible  forms  of  lenses. 
And,  lastly,  we  might  add  the  so-called  "toric"  lenses,  men- 
tion of  which  will  be  made  in  a  subsequent  chapter.  (See  p. 
115.) 

SYSTEMS    OF    LENS    MEASUREMENTS. 

Up  to  this  point  we  have  been  dealing  with  the  subject  in 
the  abstract.  To  make  the  science  of  Refraction  practical, 
it  is  necessary  to  determine  exact  values.  To  accomplish 
this  end  we  must  adopt  a  standard  of  measurements. 


LENSES.  25 

The  English  System. — Inasmuch  as  a  lens  of  great  refrac- 
tive power  causes  the  rays  of  light  to  converge  at  a  shorter 
focal  distance  than  does  one  of  less  refractivity,  the  focal  dis- 
tance expresses  the  strength  of  a  lens.  Thus,  a  spherical 
lens  with  a  focal  distance  of  ten  inches  is  twice  as  strong  as 
one  with  a  focal  distance  of  twenty  inches.  The  strength, 
likewise,  of  a  cylindrical  lens  signifies  the  distance  of  the 
focal  line  from  the  optical  centre  of  the  lens.  The  strength 
of  a  lens,  then,  may  be  expressed  in  terms  of  its  focal  dis- 
tance in  inches.  Under  the  old  English  system  this  was  the 
accepted  method.  Since  the  strength  of  a  lens  is  inversery 
proportional  to  its  focal  length,  this  method  necessitates 
the  use  of  fractions  in  making  deductions.  Consequently,  it 
is  confusing  and  laborious.  Furthermore,  the  inch  varies  in 
different  countries,  and  measurements  under  this  standard 
are  uncertain  and  scientifically  unreliable. 

THE   METRIC   SYSTEM. 

That  in  common  use  to-day  is  a  better  system,  and  has 
almost  completely  supplanted  the  inexact  and  cumbersome 
methods  of  the  past.  It  was  introduced  by  Nagel  in  1867, 
and  is  an  adaptation  of  the  metric  system  of  measurement. 
Instead  of  computing  focal  lengths,  its  values  designate  the 
refractive  power  of  a  given  lens  as  compared  with  one  of 
known  strength  adopted  as  a  standard.  This  standard  is 
a  lens  whose  focal  distance  is  one  meter  (39.36  inches),  the 
"Meterlens"  of  Xagel,  and  is  called  a  Diopter  (Monoyer). 

A  lens,  then,  having  a  focal  distance  of  one  meter  is  said 
to  have  the  refractive  power  of  one  diopter.  The  strength 
of  any  given  lens  is  expressed  in  terms  of  decimals  or  multi- 
ples of  this  standard.  For  instance,  a  lens  of  2.50  D. 
(diopters)  has  two  and  one-half  times  the  refractive  power 
of  the  standard  lens,  or  a  focal  distance  two  and  one-half 
times  shorter;  a  lens  of  .50  D.  strength  has  one-half  the  re- 
fractive power  of  the  standard,  and,  of  course,  a  focal  length 
of  two  meters. 

The    following  table   shows    the    strength   of   lenses   in 
diopters  and   their  corresponding  focal  lengths  in  English 
inches  and  in  centimeters : 
3 


26                                                       REFRACTION. 

TABLE    I. 

Diopters. 

Focal  length 
in    English 
inches. 

Focal  length 
in      c  e  n  t  i- 
meters. 

Diopters. 

Focal   length 
in    English 
inches. 

Focal   length 
in     c  e  n  t  i  - 
meters. 

0.25 

160 

400                 5.0                8 

20 

0.50 

80 

200                      5.50                   7.50 

18 

0.75                    60                   133                 6.0     • 

7 

16 

i.o                     40                   ico 

7.0 

6 

14 

1-25 

35 

80 

8.0 

5 

12.5 

1.50                        30 

66 

9.0 

4-50 

II 

1-75                        25 

57 

10.0 

4 

10 

2.O 

20 

5o 

II.  0 

3-75 

9 

2.25 

18 

44 

12.0 

3-50 

8-3 

2.50                   16 

40 

2-75                   H 

36 

14.0 

2.25 

7-i 

3.0                              12 

33 

15-0 

3.00 

6.6 

3-50                           II 

28 

16.0 

2.50 

6.2 

4.0                     10 

25 

18.0 

2.25 

5-5 

4-50 

9 

22 

20.  o  • 

.2 

5 

PRESCRIPTION   FOR   LENSES. 

For  the  purpose  of  writing  prescriptions  the  following 
signs  and  abbreviations  are  used  : 

ABBREVIATIONS    USED    IN   REFRACTION. 

V Vision  (seeing  power  of  the  eye). 

O.  D Oculus  dexter  (right  eye). 

O.  S Oculus  sinister  (left  eye). 

O.  U.  .  ...  .    .  Oculi  utrique  (both  eyes). 

S.  or  Sph Spherical  lens. 

C.  or  Cyl Cylindrical  lens. 

D Diopter. 

-J- Plus  or  Convex. 

- Minus  or  Concave. 

=         Equal  to. 

O Combined  with. 

The  prescription  "  +  1  D.  S.  C  +  .25  D.  C.  Ax.  90°,"  de- 
ciphered by  the  aid  of  this  table,  would  read:  "  Plus  one 
Diopter  Spherical  combined  with  plus  one-quarter  Diopter 
C3rlindrical  at  axis  90  degrees  "  (vertical). 

NEUTRALIZATION   OF   LENSES. 

To  gain  an  intelligent  conception  of  a  lens,  three  points 
must  be  taken  into  account,  viz.,  its  character,  its  form,  and 


LENSES.  27 

its  strength.  According  to  the  first,  a  lens  is  either  convex 
or  concave ;  its  form  determines  whether  or  not  it  is  spheri- 
cal, cylindrical  or  both;  while  its  strength  expresses  in 
diopters  its  refractive  power. 

If  a  convex  4ens  be  moved  from  side  to  side  in  front  of  the 
eye,  an  object  viewed  through  it  will  appear  to  be  in  motion, 
its  displacement  being  in  the  direction  opposite  the  move- 
ment of  the  lens.  With  a  concave  lens  the  reverse  is  true, 
the  object  apparently  moving  with  the  lens.  This  truth, 
known  in  physics  as  Parallactic  Displacement,  affords  a 
ready  means  of  determining  the  character,  kind  and  strength 
of  a  lens. 

When  the  object  viewed  through  the  lens  moves  equally 
in  all  meridians,  the  lens  is  spherical.  If  a  meridian  is  found 
at  which  there  is  no  apparent  displacement,  the  lens  is  cy- 
lindrical, and  the  meridian  producing  no  displacement  indi- 
cates the  axis  of  the  cylinder.  If  the  object  moves  in  all  direc- 
tions, but  more  in  one  direction  than  in  another,  the  lens 
examined  is  a  sphere-cylinder,  known  also  as  a  compound  or 
combination  lens.  The  same  simple  method,  therefore,  which 
served  to  determine  the  character  of  the  lens  examined  will 
also  indicate  its  kind. 

Wrhen  a  convex  and  a  concave  lens  of  equal  strength  are 
placed  together,  they  neutralize  each  other.  Moving  them 
from  side  to  side  in  front  of  the  eye  will  produce  no  ap- 
parent displacement  of  an  object  viewed  through  them.  The 
strength  of  a  lens,  therefore,  may  be  determined  by  placing 
over  it  successively  lenses  of  opposite  character  and  of  differ- 
ent degrees  of  refractive  power  until  a  lens  is  found  which 
nullifies  the  apparent  motion  of  the  viewed  object.  Know- 
ing the  focal  distance  of  each  neutralizing  lens,  the  strength 
of  the  lens  in  question  is  readily  determined  by  this  method. 

The  following  are  a  few  hypothetical  cases,  intended  to 
assist  in  mastering  this  method  of  identifying  the  kind, 
character  and  strength  of  lenses : 

(a)  On  moving  the  lens  through  different  meridians  be- 
fore the  e3Te,  objects  are  apparently  displaced  in  an  opposite 
direction  and  equally  in  all  meridians.  It  is,  therefore,  a  con- 
vex spherical  lens. 


28  REFRACTION. 

Beginning  with  the  weakest  lens  in  the  trial  case,  and 
trving  lenses  of  increasing  powers,  we  find  the  lens  in  ques- 
tion is  neutralized  by  placing  over  it  a  concave  spherical  lens 
of  1.25  diopters  strength.  The  prescription  for  the  lens 
tested  is,  therefore,  +  1.25  D.  S. 

(b)  Viewed  through  this  lens,  we  find  the  object  appar- 
ently moving  with  the  lens  in  all  meridians  except  the  hori- 
zontal.    We  have,  therefore,  a  concave  cylinder. 

Neutralizing  with  convex  cylinders  of  increasing  powers 
and  placed  horizontally,  we  find  that  all  apparent  displace- 
ment ceases  with  a  lens  of  + 1.62  diopters.  The  examined 
lens,  therefore,  is  a  -1.62  B.C.  Ax.  180°. 

(c)  The  next  lens  causes  an  apparent  movement  against 
the  lens,  more  pronounced  in  one  meridian  than  in  another. 
It  is,  therefore,  a  combination  of  a  convex  spherical  and  a 
cylindrical  lens. 

We  now  have  two  values  to  determine,  and  for  the  sake 
of  convenience,  we  begin  with  the  spherical.  It  is  found  that 
after  a  concave  spherical  lens  of  .50  D.  strength  is  placed 
over  the  lens  in  question,  there  remains  a  slight  movement, 
in  the  opposite  direction,  of  the  object  viewed  ;  a  concave 
lens  of  .75  D.  causes  a  slight  displacement  in  the  same  direc- 
tion. The  former  is,  therefore,  too  weak,  the  latter  too 
strong,  while  a  .62  D.  is  found  to  neutralize  the  lens  exactly, 
777  one  meridian.  In  the  meridian  at  right  angles  to  this  the 
object  still  continues  to  move  in  the  opposite  direction.  In- 
stead of  one  lens,  the  .62  D.,  we  now  add  concave  cylinders 
of  different  strength,  with  the  axis  of  each  coinciding  with 
the  non-refracting  meridian  of  the  unknown  lens,  until  one 
is  found  which  does  away  with  any  motion  of  the  viewed 
object.  This  result  is  met  with  a  —  1  D.  cylinder  at  axis  75°. 
The  strength  of  the  lens  in  question,  then,  is  expressed  by 
the  prescription :  +  .62  D.  S.  C  +  1  D-  C.  Ax.  75°. 

Other  Methods. — Another  method  of  recognizing  a  cylin- 
drical lens  is  by  viewing  through  it  a  straight  line,  e.g.,  a 
vertical  window  sash,  steam  pipe  or  the  edge  of  a  door.  If 
the  lens  be  rotated  in  its  plane,  that  portion  of  the  line  seen 
through  the  lens  will  be  tilted  to  a  greater  or  less  degree,  de- 
pending upon  the  strength  of  the  lens  and  the  extent  of  its 


LENSES. 


29 


rotation.  (Fig.  18.)  In  one  position  only  will  the  line  appear 
continuous,  viz.,  in  the  non-refracting  meridian  of  the  lens  or 
its  axis.  This  fact  is  utilized,  not  only  in  determining  the 
kind  of  lens,  but  also  in  estimating  its  strength,  for  having 
found  the  axis,  by  superimposing  known  cylinders  of  oppo- 
site character  with  axes  coincident,  the  lens  can  be  readily 
neutralized.  When  so  neutralized,  it  may  be  revolved  with- 


FIG.   18. — Showing  Break  in  Continuity  of  a  Line  Caused  by 
Tilting  a  Cylindrical  Lens. 

out  causing  any  break  in  the  continuity  of  the  straight  edge 
mentioned. 

Measuring-  the  Strength  01  a  Prism. — It  is  often  neces- 
sary to  determine  the  strength  and  position  of  a  simple 
prism,  or  of  a  prism  incorporated  in  a  given  lens.  A  prism 
has  no  focussing  power,  forms  no  images  and  shows  no  par- 
allactic  displacement.  That  the  given  lens  is  a  prism  or  con- 


30 


RK  PR  ACTION. 


tains  a  prism  may  be  determined  by  first  fully  neutralizing 
the  refractive  features  of  the  lens  and  then  viewing  through 
the  lens  the  edge  of  an  open  door,  or  other  surface  having  a 
straight  border.  The  portion'  of  the  straight  edge  seen 


FIG.   19. — Showing  the  Effect  of  a  Prismatic  Lens. 


through  the  lens  will  be  displaced  in  the  direction  of  the 
apex  of  the  prism.  (Fig.  19.)  By  neutralizing  with  known 
prisms,  their  bases  overlying  the  apex  of  the  unknown  lens, 


LENSES.  31 

when  the  straight  edge  appears  continuous  both  in  and  out 
of  the  lens,  the  strength  of  the  prism  has  been  determined. 
Mechanical  Devices  for  Measuring-  Lenses. — A  number  of 


FIG.  20. — Showing  Authors'  Axis  Finder. 

devices  have  been  marketed  for  the  purpose  of  facilitating 
the  neutralization  of  lenses  and  determining  with  accuracy 
the  axis  of  a  cylinder.  Such  a  device  is  illustrated  in  Fig.  20, 


FIG.  21. — Showing  a  Lens  Measure. 

and  is  known  as  an  Axis  Finder.  The  rear  aspect  shows  a 
number  of  chambers  adapted  to  lenses  of  standard  sizes, 
while  the  front  view  shows  two  springs  for  the  reception  of 


32  REFRACTION. 

neutralizing  lenses.  The  radiating  lines  indicate  the  meridi- 
ans in  degrees  of  a  circle.  When  the  unknown  lens  has  been 
properly  neutralized  the  non-refracting  meridian  of  the  neu- 
tralizing cylinder  will  be  continuous  with  one  of  these  radi- 
ating lines,  the  degree  number  indicating  the  axis  of  the 
cylinder  tested. 

The  strength  of  a  lens  may  also  be  determined  by  means 
of  a  lens  measure.  (Fig.  21. )  The  -three  pointed  rods  of  the 
instrument  are  brought  against  the  surface  of  the  lens  to  be 
tested,  the  middle  rod  lying  against  the  center  of  the  lens. 
II  either  the  instrument  or  the  lens  be  slowly  revolved  so 
that  the  three  pointed  rods  shall  successively  lie  in  each  and 
ever}T  meridian,  the  curvature,  i.  e.,  the  refractivity,  of  each 
will  be  indicated  on  the  dial.  The  surface  .of  a  spherical  lens 
is  curved  equally  in  all  meridians  and  gives  but  one  reading. 
That  of  a  cylindric  lens  is  not  curved  in  the  meridian  of  its 
axis,  while  the  meridian  at  right  angles  to  the  axis  gives  the 
greatest  reading  and  is  the  measure  of  the  strength  of  the 
C3rlinder. 

The  average  measure,  although  a  convenient  contrivance, 
is  not  entirely  accurate  in  its  results,  especially  when  testing 
lenses  of  high  refractivity. 

THE   RELATIONSHIP    OF    LENSES. 

There  exists  between  the  different  kinds  of  lenses  a  well 
defined  relationship  which  we  may  here  discuss  with  profit. 
There  is  a  similarity  of  refractive  effect  by  which  one  lens,  or 
a  combination  of  lenses,  may  sometimes  be  made  to  do  the 
work  of  another  entirely  different  lens  or  combination.  In 
studying  the  neutralization  of  lenses,  we  found  that  lens 
values  may  be  added  and  subtracted  the  same  as  any  other 
values.  Not  only  is  this  true  with  regard  to  lenses  of  the 
same  character,  as,  for  instance,  the  addition  of  a  4  1  D.  S. 
to  a  -  .50  D.  S.  making  a  +  .50  D.  S.,  or  a  -  1.50  D.  S.  to  a 
f  2.50  D.  S.  making  a  -f  4  D.  S.,  but  also  lenses  of  different 
character  may  be  added  or  subtracted  in  a  similar  manner. 
A  -  .50  D.  Cyl.  may  be  added  to  a  +  .50  D.  S.,  leaving  a 
f  .50  D.  Cly.  at  the  opposite  axis.  We  will  consider,  first : 

The  Relationship  Between  Spheric  and  Cylindric  Lenses. 


LENSES.  33 

— A  cylindric  lens  of  any  given  strength  has  just  one-half  the 
total  refraction  of  a  spherical  lens  of  the  same  strength 
or  number.  This  will  at  once  become  clear  if  we  consider 
even-  spherical  lens  as  composed  of  two  cylindrical  lenses  of 
the  same  character  and  number  with  their  axes  at  right 
angles  to  each  other.  Thus  a  +  1  D.  spherical  may  be  con- 
sidered as  made  up  of  a  -h  1  D.  cylinder  at  one  axis  with 
a  r  1  D.  cylinder  at  right  angles  to  the  former.  Likewise, 
if  we  place  over  a  -r  1  D.  spherical  lens  a  —  1  D.  cylindrical, 
at  the  horizontal  axis,  what  have  we  left?  A  -f  1  D.  cylin- 
der at  the  vertical  axis ;  in  other  words,  we  have  neutralized 
one  of  the  two  cylinders  of  which  the  sphere  was  composed 
and  we  have  left  the  other  component  cylinder.  It  follows 
as  a  self  evident  fact,  that  in  adding  two  cylinders  of  like 
characters,  but  of  opposite  axes  and  different  powers,  to 
form  a  sphero-cylindrical  lens,  that  the  weaker  cylinder 
determines  the  strength  of  the  sphere,  the  remainder  of  the 
stronger  cylinder  retaining  its  identity.  Thus:  -f  1  D.  C. 
Ax.  90°  3  +  .75  D.  C.  Ax.  180°  ==  +  .75  D.  S.  C  -  -25  D.  C. 
Ax.  90°. 

To  become  proficient  in  lens  computations  it  will  be  well 
for  the  student  to  work  out  a  number  of  different  examples 
similar  to  the  following: 

Example:  +  1.25  D.  S.  -  .50  D.  C.  Ax.  180°  =  =  +  (1.25D. 
C.  Ax.  90°)  +  (1.25  D.  C.  Ax.  180°)  -  .50  D.  C.  Ax.  180°  = 
-r  1.25  D.  C.  Ax.  90°  +  .75  D.  C.  Ax.  180°.     R  +  .75  D.  S.  C 
•f  .50  D.  C.  Ax.  90°. 

But  the  relationship  between  spheric  and  cylindric  lenses 
is  even  closer,  for  the  effect  of  a  spherical  lens  placed  ob- 
liquely in  the  path  of  luminous  rays  is  actually  identical 
with  that  of  a  cylinder.  In  fact,  Young,  who  was  the  dis- 
coverer of  ocular  astigmia,  believed  the  cylindric  effect 
(Astigmia)  of  the  refracting  media  of  the  eye  to  be  due 
wholly  to  the  oblique  position  of  the  crystalline  lens.  This 
cylindric  effect  increases  with  the  degree  of  inclination  of  the 
lens. 

The  Relationship  of  Cylinders. — If  a  +  2  D.  cylinder  and  a 
—  2  D.  cylinder  are  so  placed  that  their  axes  correspond  they 
will  neutralize  each  other.  When,  however,  they  are  turned 
about  their  centres  so  that  their  axes  are  at  right  angles  to 


34 


REFRACTION. 


each  other,  they  vary  constantly  in  their  respective  refrac- 
tive powers,  as  the  two  cylinders  approach  right  angles. 
When  they  lie  in  opposite  meridians  they  exercise  a  combined 
cylindric  effect  equal  to  the  sum  of  the  power  of  the  two 
lenses,  viz.,  4  D. 

The  Relationship  of  Spheres  to  Prisms. — A  spherical  lens 
is  theoretically  composed  of  a  large  number  of  prisms,  base 
in  or  out,  according  as  the  lens  is  convex  or  concave.  This 
was  mentioned  and  illustrated  when  considering  spherical 
lenses  (q.  v.). 

There  exists,  however,  a  practical,  as  well  as  a  theoretical 
relationship  between  spherical  and  prismatic  lenses. 

Between  the  optical  centre  of  a  spherical  lens  and  the 
periphery,  the  refractive  effect  is  that  of  a  sphere  with  the 
addition  of  a  prism.  This  phenomenon  is  most  apparent  in 
a  lens  in  which  the  thickest  or  thinnest  point  does  not  lie 
equi-distant  from  the  periphery.  Such  a  lens  is  said  to  be 
decentered.  A  glance  at  the  accompanying  Fig.  22  shows 


FIG.  22. — Showing  Prismatic  Effect  of  Decentering  a  Lens. 

a  section  of  a  decentered  lens  which  is  seen  to  be  precisely 
the  same  as  though  a  normally  centered  lens  had  been  split 
flatwise  and  a  prism  had  been  introduced  between  the 
halves.  The  strength  of  the  added  prismatic  effect  depends, 
of  course,  upon  the  strength  of  the  lens  and  the  extent  to 
which  it  has  been  decentered.  This  relationship  has  been 
accurately  determined  and  tabulated  and  may  be  found  in 
detail  in  some  of  the  more  pretentious  works  on  Refraction. 
The  Relationship  of  Cylinders  to  Prisms. — A  cylindrical 
lens,  or  the  cylindrical  element  of  a  sphero-cylindrical  lens, 
when  decentered  in  a  direction  vertical  to  its  axis,  shows 
the  prismatic  effect  of  a  spherical  lens  of  the  same  strength. 


CHAPTER  III. 
THE  NORMAL  EYE. 

INTRODUCTION. 

The  percentage  of  eyes  approaching  more  or  less  closely 
to  a  condition  which  would  be  counted  as  normal  is  rela- 
tively small  (not  more  than  2  to  10  per  cent.),  yet  it  will  be 
necessary  to  consider  the  optical  properties  of  such  an  eye 
before  entering  upon  a  discussion  of  abnormal  conditions. 
As  health  and  disease  are  relative  terms  there  must  be  a 
standard  upon  which  to  base  our  comparisons  and  compu- 
tations. Emmetropia  is  the  ideal  state  of  refraction  and  the 
Emmetropic  eye  is  one  of  perfect  morphology  and  function. 
In  such  an  eye  parallel  rays  of  light  fall  exactly  upon  the 
retina  without  effort  of  accommodation.  The  Emmetropic 
eye,  then,  is  the  end  and  aim  of  the  refractionist. 

REQUISITES     OF    VISION. 

In  the  emmetropic  eye  the  three  requisite  conditions  to 
perfect  vision  are  met,  viz.,  1.  The  pupillary  aperture  is  free 
from  all  obstructions  and  the  media  are  clear  and  trans- 
parent; 2.  The  refracting  surfaces  are  such  as  to  focus 
parallel  rays  exactly  upon  the  retina ;  3.  The  retina  is -suffi- 
ciently sensitive  to  perceive  the  image  created. 

DIOPTRIC    SYSTEM   OF    THE  EYE. 

We  shall  here  consider  briefly  the  individual  factors  enter- 
ing into  the  Dioptric  System,  or  optical  mechanism  of  such 
an  eye.  Primarily  the  dioptric  system  is  composed  of  the 
Cornea,  Aqueous  Humor,  Crystalline  Lens  and  Vitreous. 
Since  the  two  surfaces  of  the  cornea  are  practical!}'  parallel 
and  its  index  of  refraction  is  the  same  as  that  of  the  aqueous, 
these  two  media  may  be  considered  as  one.  The  anterior 
surface  of  the  cornea  is  the  most  active  of  all  the  refracting 
surfaces  of  the  dioptric  system,  because  the  difference  be- 


36  REFRACTION. 

tween  the  refracting  indices  of  air  and  the  aqueous  humor 
is  greater  than  that  of  any  two  contiguous  media  in  the  in- 
terior of  the  eye  (see  Optical  Principles). 

The  Crystalline  Lens  is  not  homogeneous,  but  consists  of 
a  spherical  nucleus  having  a  very  high  refractive  index  and 
of  a  great  number  of  superimposed  layers  whose  curvature 
and  refractive  index  decrease  from  within  outward.  This 
structure  has  the  effect  of:  1st,  Diminishing  the  spherical 
aberration  and,  2d,  Giving  the  lens  a  greater  refractive 
power  than  wroulcl  be  possible  with  a  homogeneous  body  of 
the  same  shape  (Landolt).  The  vitreous  is  a  homogeneous 
medium,  the  refractive  index  of  which  is  the  same  as  that  of 
the  cornea  and  aqueous. 

STATIC   REFRACTION. 

Acting  together  the  dioptric  surfaces  effect  the  Static  Re- 
fraction of  the  eye,  producing  distinct  retinal  images,  with 
the  optical  mechanism  in  a  state  of  repose.  The  pupil  maybe 
slightly  contracted,  but  otherwise  muscular  activity  is  sus- 
pended and  no  effort  is  needed  for  distinct  vision.  The  visual 
axes  are  practically  parallel  and  the  eye  is  focussed  for  its  far 
point  or  Punctum  Remotum  (which  in  the  emmetropic  eye 
lies  at  infinity). 

DYNAMIC   REFRACTION. 

While  the  eye  is  focussed  for  distant  vision  a  near  object 
will  appear  indistinct.  To  see  this  near  object  clearly,  the 
distance  between  the  observer  and  the  object  viewed  re- 
maining constant,  it  is  necessary  to  change  the  focus.  A 
photographer  has  to  focus  his  camera  in  order  to  obtain 
a  clearly  defined  image  on  his  plate.  To  do  this  he  has  the 
choice  of  four  methods  of  procedure,  viz.:  1.  To  move  the 
object  viewed,  the  lens  and  plate  remaining  stationary;  2. 
To  move  the  lens,  the  object  and  plate  remaining  station- 
ary; 3.  To  move  the  plate,  the  object  and  lens  remaining 
stationary ;  4.  To  increase  or  decrease  the  refractive  power 
of  the  lens. 

In  the  human  eve,  inasmuch  as  the  lens  and  retina  are 
normally  immovable,  only  two  of  the  four  means  of  focuss- 


THE   NORMAL  EYE.  37 

ing  are  possible.  By  approaching  toward,  or  receding  from 
the  object  viewed,  and  more  especially  by  increasing  to  a 
greater  or  less  degree  the  refractivity  of  the  lens,  the  diverg- 
ing rays  of  objects  less  than  twenty  feet  distant  are  brought 
to  a  focus  on  the  retina.  This  power  of  the  eye  to  adjust 
itself  for  near  vision  is  known  as  the  Dynamic  Refraction  or 
Accommodation.  When  exercised  to  its  highest  degree  the 
accommodation  focusses  the  eye  for  its  near  point,  or 
Punctum  Proximum.  The  difference  in  the  refractivity  of  the 
eye  during  maximum  and  minimum  accommodation  is 
called  the  range  or  Amplitude  (Bonders)  of  Accommodation 
and  is,  of  course,  expressed  in  terms  of  diopters. 

PHYSIOLOGY   OF  ACCOMMODATION. 

Two  factors  enter  into  the  accommodation  of  the  eye, 
viz.:  1.  The  normal  resiliency  of  the  Lens,  and  2,  the  action 
of  the  Ciliary  Body. 

We  are  indebted  to  Helmholz  for  our  knowledge  of  the 
physiology  of  accommodation  and  so  complete  were  his  re- 
searches as  to  leave  little  for  modern  investigators  to  add. 
The  Ciliary  Body  consists  essentially  of  two  sets  of  muscles, 
one  of  which  constitutes  a  sphincter  (annular  muscle  of 
Miiller),  the  other  having  a  radial,  longitudinal  arrange- 
ment and  tying  parallel  with  the  sclerotic.  These  fibres  ex- 
tend back  into  the  choroid  and  have  as  their  fixed  point  of 
attachment  the  corneo-scleral  junction.  The  suspensory 
ligament  of  the  lens,  or  Zonule  of  Zinn,  is  attached  to  the 
Ciliar\r  Body  under  tension  so  that  when  this  ligament  is 
cut  the  lens,  left  to  its  own  elasticity,  becomes  more  convex. 

The  same  phenomenon  occurs  by  the  action  of  the  ciliary 
muscles  as  follows :  When  the  lens  is  released  from  the  ten- 
sion of  the  Zonule  of  Zinn  by  the  contraction  of  the  sphincter 
fibres  principally,  the  lens  bulges.  Both  surfaces  increase 
their  convexity,  the  anterior  more  than  the  posterior. 
(Fig.  23.) 

CONVERGENCE. 

To  secure  binocular  vision  for  near  objects,  it  is  not  only 
necessary  that  the  focus  be  regulated,  but  that  the  eyes 


38 


FIG.  23. — Showing  Change  in  the  Shape  of  the  Lens  During  Accommoda- 
tion.— (After  Landolt. ) 


THE  NORMAL   EYE.  39 

should  rotate  inward,  the  degree  of  rotation  depending 
upon  the  nearness  of  the  object.  This  phenomenon  is  known 
as  Convergence.  In  the  exercise  of  this  function  of  the  eye, 
the  fovea  centralis,  which  is  the  most  sensitive  portion  of 
the  retina,  is  so  directed  toward  an  object  that  the  image  is 
formed  upon  it.  The  eye  is  then  said  to  "fix"  the  object; 
the  images  formed  in  the  two  eyes  fall  upon  identical  points 
and  fuse  so  that  the  brain  perceives  but  one.  Should  the 
images  fall  upon  corresponding  points  above,  below,  to 
the  right  or  left  of  the  fovea  they  should  likewise  produce 
but  a  single  impression.  If,  however,  the  image  should  fall 
upon  points  bearing  a  different  relation  to  the  fovea,  as 
would  be  the  case  if  the  function  of  convergence  were  sus- 
pended or  improperly  performed,  both  images  would  be  per- 
ceived. In  other  words,  we  would  have  a  condition  known 
as  Diplopia  or  double  vision. 

For  binocular  distant  vision  the  axes  of  the  eyes  must  be 
parallel.  For  binocular  vision  at  distances  less  than  twenty 
feet  the  Rectus  Internus  muscle  draws  each  eye  inward  to  a 
degree  inversely  proportional  to  the  distances  of  the  object 
looked  at.  To  measure  the  convergence  intelligently  we  em- 
ploy as  a  unit  the  angle  through  which  the  visual  axis  of  the 
eye  moves  to  fix  on  a  point  one  meter  distant — the  meter- 
angle  of  convergence  (Nagel).  If  the  object  viewed  be  only 
one-half  meter  distant  the  amount  of  convergence  necessary 
will  be  two  meter-angles,  and  conversely  at  two  meters  dis- 
tance the  convergence  would  be  one-half  meter-angle.  The 
same  number  which  expresses  the  diopters  of  accommoda- 
tion necessary  for  distinct  near  vision  also  expresses  the 
amount  of  convergence  in  meter-angles.  For  when  the  ob- 
ject viewed  is  one-half  meter  distant  and  the  convergence  is 
two  meter-angles  the  amount  of  accommodation  brought 
into  play  is  two  diopters,  and  again,  conversely,  at  two 
meters  distance  the  amount  of  convergence  is  one-half  meter- 
angle  and  the  accommodation  required  one-half  D. 

The  amplitude  of  convergence  is  the  number  of  meter- 
angles  of  convergence  of  which  the  eyes  are  capable  in  pass- 
ing from  a  condition  of  complete  relaxation — far  point  of 
convergence — to  one  of  maximum  convergence  or  the  near 
point. 


40  REFRACTION. 


RELATIONSHIP    OF    ACCOMMODATION,   CONVERGENCE    AND     PU- 
PILLARY  CONTRACTION   IN    THE    EMMETROPIC    EYE. 

From  the  above  facts  it  will  be  seen  that  accommodation 
and  convergence  are  very  closely  associated,  inasmuch  as 
both  increase  and  decrease  together  and  to  the  same  degree. 
This  relationship,  however,  is  not  absolute,  for  it  is  possible 
for  the  eye  to  fix  upon  a  given  point  and  with  convergence 
stationary  to  increase  or  decrease  its  accommodation  and, 
conversely,  to  focus  for  a  given  distance  and  without  alter- 
ing this  focus,  to  increase  or  decrease  the  convergence  angle. 
The  former  is  known  as  the  relative  amplitude  of  accommo- 
dation, the  latter  as  the  relative  amplitude  of  convergence. 

Throughout  the  physiological  range  of  accommodation 
and  convergence  in  the  emmetropic  eye  their  relationship 
may  be  said  to  be  absolute,  beyond  this  limit,  relative.  For, 
when  the  eye  is  adjusted  for  its  Punctum  Remotum,  the  ac- 
commodation may  be  relaxed  still  more,  and  when  the  ac- 
commodation is  at  its  maximum,  the  amount  of  convergence 
associated  is  not  only  relatively  greater,  but  is  capable  of  a 
considerable  increase.  Advancing  age,  as  will  be  shown 
hereafter,  diminishes  the  power  of  accommodation  to  a  point 
of  total  suspension,  yet  has  no  effect  upon  the  function 
of  convergence. 

The  Iris  plays  an  unimportant  role  in  connection  with  the 
accommodation,  in  fact,  this  membrane  may  be  entirely 
wanting  without  greatly  disturbing  the  power  of  accommo- 
dation. Normally,  however,  the  pupil  contracts  during  ac- 
commodation, and  dilates  when  the  latter  is  relaxed.  The 
nerves  supplying  the  ciliary  muscle  belong  to  the  Motor 
Oculi,  which  contain  also  the  fibres  distributed  to  the 
sphincter  of  the  pupil.  Their  origin  seems  to  be  in  the  floor 
of  the  fourth  ventricle.  Irritation  of  the  anterior  .portion  of 
the  floor  produce  s  accommodation  ;  irritation  farther  back 
causes  contraction  of  the  pupil  and,  when  that  part,  where 
the  fourth  ventricle  passes  into  the  Aquasductus  Sylvii,  is  ir- 
ritated, contraction  of  the  Rectus  Internus  results.  Gencrallv 
speaking,  pupillary  contraction  is  more  closely  associated 


THE   NORMAL   EYE. 


41 


with  convergence  than  with  accommodation,  for  it  takes 
place  with  convergence  in  the  absence  of  accommodation. 
The  following  table  will  serve  as  a  summary  of  these  facts : 

TABLE  II. 

SHOWING   THE    RELATIONSHIP    OF    ACCOMMODATION,    CONVERGENCE   AND 
PUPILLARY    CONTRACTION. 


The  Eyes. 


Accommodation. 


Convergence. 


Pupillary  Action. 


When  adapted  Relaxed,  but  has  Completely  relax- 
to  the  Punc-  some  reserve  ca-  ed.  Visual  axes 
turn  Remotum  pacity.  parallel. 

When  adapted  Maximum  power  E  x  e  re  i  se  d  to  a 
to  Punctum  exerted.  greater  degre  e 

Proximum.  than  is  accommo- 

dation   and    can 
be  increased. 


Pupil  dilated. 


Pupil  contracted. 


Under   Mydria-  Relaxed, 
tics. 

Under  Myotics.   Increased. 


Activity  decreased.  Pupil  dilated. 


Possible  action  in- 
creased. 


Advancing  Age.   Decreased     to     a  Not  affected, 
point  of  total  sus-  | 
pension. 


Pupil  contracted. 


Pupil  contracted. 


OPTICAL   DEFECTS   OF  THE   NORMAL   EYE. 

We  have  spoken  of  the  Emmetropic  eye  as  one  of  perfect 
morphology  and  function,  yet  this  is  a  theoretical  concep- 
tion. Such  an  eye  does  not  exist  and,  from  a  practical  stand- 
point, instead  of  associating  the  idea  of  "  perfection  "  with 
the  term  Emmetropia,  we  must  be  content  to  accept  the  Em- 
metropic eye  as  the  "normal"  eye,  the  standard  by  which 
we  are  to  judge  anomalies.  This  normal  eye,  although  the 
most  wonderful  of  all  the  organs  of  the  body,  the  most  intri- 
cate and  complex  in  its  construction,  marvelous  in  its  detail 
and  delicate  in  its  adjustment,  nevertheless  shows  many 
glaring  defects.  "Many  of  these,"  says  Bid  well,  "are  the 
more  striking  because  they  are  so  obvious.  The  external  sur- 
face of  the  lens  formed  by  the  aqueous  humour  and  the 
4 


42  REFRACTION. 

cornea  is  not  a  surface  of  revolution,  such  as  would  be  fash- 
ioned  by  a  turning  lathe  or  a  lens  grinding  machine;  its 
curvature  is  greater  in  a  vertical  than  in  a  horizontal  direc- 
tion, and  the  distinctness  of  the  focussed  image  is  conse- 
quently impaired.  Again,  the  crystalline  lens  is  constructed 
of  a  number  of  separate  portions  which  are  imperfectly 
joined  together.  Striae  occur  along  the  junctions,  and  the 
light  which  traverses  them  instead  of  being  uniformly  re- 
fracted is  scattered  irregularly.  Moreover,  the  system  of 
lenses  is  not  centered  upon  a  common  axis;  neither  is  it 
achromatic,  while  the  means  employed  for  correcting  spheri- 
cal aberration  are  inadequate." 

These  defects,  being  inherent  in  the  design  or  structure  of 
the  eye  itself,  produce  anomalies  which  we  may  classify  as 
physical  in  comparison  with  those  of  psychic  origin.  The 
latter  result  from  the  erroneous  interpretations  placed  by 
the  mind  upon  the  phenomena  presented  to  it  through  the 
medium  of  the  optic  nerve  and  the  brain.  Under  this  head 
may  be  classified  the  many  strange  manifestations  known 
as  "Optical  Illusions." 

In  spite  of  the  many  defects  set  forth,  however,  our  eyes 
do  us  excellent  service  because  with  incessant  practice  we 
have  acquired  a  very  high  degree  of  skill  in  their  use.  Bid- 
well  sa37s,  tersely:  "If  anything  is  more  remarkable  than 
the  ease  and  certainty  with  which  we  have  learned  to  in- 
terpret ocular  indications  when  they  are  in  some  sort  of 
conformity  with  external  objects,  it  is  the  pertinacity  with 
which  we  refuse  to  be  misled  when  our  eyes  are  doing  their 
best  to  deceive  us." 

CHANGES  OCCURRING   DURING  THE  LIFE  HISTORY  OF  THE  EYE. 

In  addition  to  the  ocular  defects  enumerated  there  must 
be  mentioned  certain  changes  occurring  in  the  eye  at  differ- 
ent periods  of  its  life  history,  changes  which  are  prejudicial 
to  good  vision.  By  reason  of  the  regularity  and  uniformity 
with  which  these  changes  take  place,  they  are  looked  upon 
as  physiological,  and  are  necessarily  incident  to  the  two 
periods  of  life,  viz.,  that  of  growth  and  development  and 
that  of  decline  (senility).  When  we  consider  that  the  eye 


THE   NORMAL   EYE.  43 

continues  to  develop  for  many  years  and  that  throughout 
this  growth  a  very  accurate  and  definite  relationship  must 
be  maintained  between  its  component  parts,  we  can  readily 
appreciate  the  wide  range  of  possible  refractive  errors  to 
which  the  dioptric  system  is  subjected.  And  when  we  take 
cognizance  of  the  many  and  varied  uses  and  abuses  to  which 
the  eyes  are  subjected  during  the  life  of  the  individual,  the 
frequency  of  refractive  errors  ceases  to  be  an  object  of 
wonder. 

Nearly  all  eyes  are  far-sighted  at  birth.  This  is  owing  to 
the  fact  that  the  eye-ball  has  not  reached  its  full  size  and  is, 
therefore,  too  short  to  bring  parallel  rays  to  a  focus  on  the 
retina.  In  this  condition,  known  as  Axial  Hyperopia,  or 
far-sightedness,  parallel  rays  focus  too  far  behind  the  retina. 
Not  only  is  this  condition  found  to  exist  in  the  new-born, 
but  it  continues  for  a  longer  or  shorter  period  and  is  still 
the  rule  at  ten  years,  although  the  power  of  accommoda- 
tion, which  has  a  wider  range  of  activity  in  youth  than  at 
any  other  time  of  life,  usually  serves  to  overcome  the  anom- 
aly in  whole  or  in  part. 

The  pupil,  which  is  very  much  contracted  in  new-born  in- 
fants, soon  becomes  more  dilated,  smaller  again  in  adult  life 
and  still  more  contracted  in  old  age.  Its  activity  is  greatly 
lessened  in  the  decline  of  life,  owing  to  the  unyielding  char- 
acter of  the  tissues  of  the  iris,  especially  that  of  the  Sphincter 
Pupillae. 

The  accommodation  of  the  eye  shows  a  steady  decline 
throughout  life.  The  lens  apparently  partakes  of  the  general 
tendency  of  the  tissues  of  the  body  to  become  more  rigid 
and  unyielding  as  the  age  of  the  individual  increases  and 
hence  the  degree  of  convexity  caused  by  the  action  of  the 
ciliary  muscle  is  proportionately  decreased.  While  this  is 
undoubtedly  the  prime  factor  in  causing  the  functional  de- 
cline of  the  accommodative  mechanism,  the  laxity  of  fibre 
and  muscular  atrophy  incident  to  old  age  must  also  be  con- 
sidered as  pla}ring  an  important  role. 

This  gradual  failure  of  the  focussing  power  of  the  eye, 
known  as  Presbyopia,  bears  a  very  definite  relationship  to 
the  age  of  the  individual.  Table  4,  page  86. 


44 


REFRACTION. 


VISUAL   ACUITY. 

In  order  that  an  object  may  be  distinctly  seen  by  the 
normal  eye,  it  must  be  of  such  size  as  to  subtend  an 
angle  of  at  least  one  minute,  the  eye  being  at  the  point 
of  the  angle.  This  fact  is  utilized  in  the  construction  of 
the  Snellen  test  types,  the  limbs  of  which  subtend  an 
angle  of  one  minute  and  the  whole  letter  an  angle  of  five 
minutes.  (Fig.  24.)  The  size  of  the  letter  or  object  included 


FIG.  24. — Showing  Construction  of  Snellen's  Test  Letters. 

in  an  angle  of  five  minutes  is,  of  course,  constant  for  a  given 
distance,  increasing  as  it  recedes  from  and  decreasing  as  it 
approaches  nearer  the  eye.  (Fig.  25.)  A  Snellen  test  card 


FIG.  25. — Showing  Principle  Involved  in  Snellen's  Letters. 

contains  a  number  of  letters  of  varying  sizes  arranged  in 
rows,  each  row  bearing  a  number  expressing  the  distance  in 
feet  or  meters  at  which  its  letters  subtend  an  angle  of  five 


THE   NORMAL   EYE. 


45 


minutes,  hence  the  distance  at  which  they  should  be  read  by 
the  normal  eye.     (Fig.  26.) 

Sea  ting  a  patient  at  a  distance  of  twenty  feet  from  the  test 
card,  (Fig.  27)  (this  distance  being  chosen  because  at  this 
range  rays  of  light  are  practically  parallel),  and  testing  one 
eye  at  a  time,  the  lowest  line  the  patient  is  able  to  read  cor- 
rectly is  the  measure  of  his  visual  acuity.  This  is  recorded 
in  terms  of  a  fraction,  the  numerator  of  which  expresses  the 
distance  of  the  patient  from  the  test  card  and  the  denomi- 
nator the  number  above  the  lowest  line  read.  Hence  V  !&, 


^^ 

C 

R   B 
T  r  P 

5    C    C    O 

4     K     B     E     R 
3    v    y    r    P    T 


FIG.  26. — Showing  Test  Card. 

would  signify  that  at  20  feet  the  patient  reads  the  line  which 
he  should  be  able  to  read  at  40  feet.  Visual  acuity  of  f$  should 
be  looked  upon  as  the  minimum  of  the  normal  adult  eye,  most 
eyes  of  young  people  having  an  acuity  of  f f  and  even  f$. 
When  the  acuity  falls  below  f£,  except  in  aged  individuals, 
the  eye  is  no  longer  to  be  considered  normal. 

In  testing  the  acuity  of  vision  the  best  possible  illumina- 
tion is  necessary.  When  the  test  is  made  on  cloudy  days  due 
allowance  must  be  made  for  the  apparent  decrease  in  vision. 


46 


REFRACTION'. 


Tests  may  be  conducted  in  a  subdued  light  by  means  of 
porcelain  test  cards  bearing  transparent  letters  which  are 
illuminated  from  the  rear. 


FIG.  27.— Showing  Changeable  Test  Card. 


CHAPTER  IV. 
ASTHENOPIA. 

Under  the  term,  Asthenopia,  may  be  grouped  the  many 
and  varied  subjective  symptoms  complained  of  by  patients 
suffering  from  some  error  of  refraction.  The  word  signifies 
"painful  vision;"  the  underlying  cause  of  the  condition 
being  exhaustion  of: 

(a)  The  Ciliary  Muscle,  producing  what  may  be  called 
Accommodative  Asthenopia,  or 

(b)  The  Internal  Recti  Muscles,  producing  Muscular  or 
Convergence  Asthenopia,  or 

(c)  A  combination  of  both  (a)  and  (b). 

In  the  first  form,  the  Ciliary  Muscle  is  overtaxed  by  its 
efforts  to  overcome  some  refractive  error.  In  the  second,  the 
Internal  Recti  Muscles  are  unequal  to  the  task  imposed  on 
them.  This  failure  of  perfect  function  may  be  due  to  con- 
genital weakness  of  the  muscles ;  or,  too  wide  separation  of 
the  eyes  necessitates  an  excessive  amount  of  convergence  in 
order  to  bring  each  eye  into  perfect  alignment.  In  the  third 
form  there  is  a  combination  of  both  these  conditions  and 
consequently  a  doubled  cause  for  painful  vision. 

SYMPTOMS. 

The  more  common  symptoms  complained  of  are  head- 
ache, pain  at  the  back  of  the  eyes  and  around  the  eyes,  any 
of  the  symptoms  usually  termed  ''neuralgia"  of  the  head  or 
face,  conjunctival  irritation,  photophobia,  lachrymation, 
spasm  of  the  lids,  blepharitis,  apparent  movement  of  objects 
steadily  gazed  at,  blurring  of  vision,  running  together  of  the 
type,  feeling  of  dryness  of  the  lids,  sensation  of  sand  in  the 
eyes  requiring  the  patient  to  rub  them  for  temporary  relief, 
dizziness,  and  even  pronounced  functional  nervous  disturb- 
ances. While  they  vary  greatly  with  the  patient's  general 
health,  all  of  the  symptoms  are  usually  worse  after  a  day's 


48  REFRACTION. 

work,  or  on  first  waking  in  the  morning.  Sometimes  a 
marked  error  of  refraction  becomes  manifest  only  as  a  sequel 
to  some  exhausting  illness. 

The  symptoms  induced  by  hyperopia  usually  follow  ex- 
cessive use  of  the  eyes  for  close  -work,  such  as  reading,  writ- 
ing, drawing,  sewing,  etc.,  while  those  of  myopia  are  more 
commonly  induced  when  the  patient  uses  his  eyes  excessively 
for  distant  vision.  The  strain  caused  by  astigmia  is  always 
present,  although  it  usually  becomes  more  manifest  after 
the  excessive  convergence  and  accommodation  incident  to 
near  work. 

ASTHENOPIA   AS  A   CAUSE   OF   GENERAL   DISEASE. 

Much  has  been  written  on  the  subject  of  asthenopia  as  a 
cause  of  general  disease.  The  argument  is  based  on  the  fact 
that  in  eye  strain  the  constant  expenditure  of  nerve  energy, 
due  to  the  patient's  efforts  to  see  distinctly,  results  in  nerv- 
ous exhaustion.  It  certainly  seems  reasonable  that  a  pro- 
nounced error  of  refraction  when  uncorrected,  especially  in 
patients  who  make  more  than  ordinary  demands  upon  their 
eyes,  like  any  other  source  of  continued  irritation,  may  so 
far  deplete  the  economy  as  to  render  it  liable  to  the  invasion 
of  disease  which,  under  normal  conditions,  would  be  success- 
fully combated  by  the  protective  forces  of  the  body.  Every 
physician  -will  testify  to  pronounced  cases  of  general  dis- 
turbance which  resisted  all  forms  of  treatment  until  the 
error  of  refraction  or  the  muscular  imbalance  of  the  eyes 
had  been  corrected. 


CHAPTER  V. 
THE  TRIAL  CASE  AND  ITS  USES. 

INTRODUCTION. 

The  trial  or  test-case  consists  of  an  assortment  of  convex 
and  concave  spherical  and  cylindrical  lenses.  (Fig.  28.) 

Spherical  Lenses. — The  spherical  lenses  are  arranged  in 
two  double  rows,  convex  on  the  right  side  of  the  case,  con- 
cave on  the  left.  Between  these  rows  is  a  double  column  of 


FIG.  28. — Showing  Trial  Case. 

figures,  one  denoting  the  strength  of  the  lenses  opposite  in 
terms  of  Diopters,  the  other  specifying  the  focal  distance  in 
inches.  In  a  full  case  these  lenses  range  from  .12  D.  to  2O  D., 
the  weakest  lenses  occupying  the  front  of  the  case.  Each 
lens  has  stamped  upon  its  handle,  or  cut  into  the  glass,  the 
figure  expressing  its  strength  in  diopters. 

Cylindrical  Lenses. — The  cylindrical  lenses  are  similarly 
arranged  in  double  rows  in  the  center  of  the  case,  convex  on 


50  REFRACTION. 

the  right,  concave  on  the  left,  and  are  numbered  in  like  man- 
ner. They  are  usually  fewer  in  number  than  the  spherical 
lenses.  In  the  edge  of  the  lens  a  little  cut  into  the  glass  in- 
dicates the  axis  of  the  cylinder.  Each  one  is  so  marked,  or 
the  lens  has  a  segment  at  each  side  rendered  opaque  by 
grinding  the  glass,  the  edge  of  the  opacity  corresponding  to 
the  axis  of  the  cylinder.  (Fig.  29.) 

Prismatic  Lenses. — Prismatic  lenses  usually  occupy  a  sep- 
arate compartment  at  the  back  of  the  case.  They  are  num- 
bered from  1  to  20  to  express  in  degrees  the  angle  formed  by 
their  two  sloping  surfaces. 


FIG.  29. — Showing  Cylindrical  Test  L,ens. 

There  are  in  addition  a  number  of  accessories,  as  follows  : 

(a)  An  obturator,  an  opaque  disc  made  of  metal  or  hard 
rubber,  used  to  cover  one  eye  when  examining  the  other. 

(b)  Ground  glass  disc  used  for  the  same  purpose. 

(c)  Pin-hole  disc,  a  metal  or  hard  rubber  disc  having  in 
its  center  a  minute  aperture.     (Fig.   30.)     It  has  the  effect, 
when  placed   before  the  eye,  of  cutting  off  the  peripheral 
rays,  thereby  reducing  the  size  but  sharpening  the  outline  of 
the  image  produced  on  the  retina.    When  vision  can  be  im- 
proved in  this  manner  it  can  also  be  improved  -by  lenses. 


THE   TRIAL   CASE   AND   ITS   USES. 


51 


(d)  Stenopseic  disc. — This  is  an  opaque  disc  in  the  center 
of  which  is  cut  a  narrow  slit.  (Fig.  31.)  It  is  used  in  testing 
for  astigmia  (q.  v.). 


FIG.  31. — Showing   Stenop- 


FiG.  30. — Showing  Pin  Hole 
Disc. 


(e)    Maddox  rod. — This  is  an  opaque  disc   with  one  or 
more  parallel  stenopseic  slits,  in  front  of  each  of  which  and 


FlG.  32. — Showing  Maddox 
Multiple  Rod. 


FlG.  33. — Showing  Double 
Prism. 


parallel  to  it  is  a  glass  rod  or  cylinder.     (Fig.  32.)     A  flame, 
when  looked   at  through  the  maddox  rod,  appears  as  a 


52 


REFRACTION. 


luminous  streak  passing  through  the  center  of  the  cylinders 
and  at  right  angles  to  them.  Its  use  will  be  explained  in  dis- 
cussing the  subject  of  muscular  imbalance. 

(f)  Double  prism.— This  is  a  disc,  at  the  centre  of  which 
are  two  prisms,  base  to  base.  (Fig.  33.)  Objects  looked  at 
through  these  prisms  will  appear  double.  Its  use  will  be  ex- 
plained in  the  study  of  muscular  imbalance. 

The  Trial  Frame. — The  trial  frame  is  a  contrivance,  re- 
sembling n  pair  of  crude  spectacles,  Fig.  34,  used  in  conduct- 
ing a  test  of  vision.  It  has  a  lens  chamber  for  each  eye,  each 
chamber  being  capable  of  receiving  two  or  three  lenses. 
These  mav  be  revolved  so  that  the  cvlindrical  lenses  mav  be 


FIG.  34.  — Showing  Trial  Frame. 

placed  at  any  axis  from  0  to  180  degrees.  The  angle  of  the 
cylindrical  axis  is  indicated  in  degrees  of  the  circle  marked  on 
a  celluloid  arc  above  the  lens  chamber.  The  trial  frame  is  so 
arranged  that  it  can  be  adjusted  to  suit  the  patient's  nose, 
pupillary  distance,  length  of  head  and  width  of  face. 
(Fig.  35.)  In  addition  to  the  features  common  to  all,  the 
authors'  trial  frame,  Fig.  36,  is  fitted  with  a  temple  adjust- 
ment by  which  the  frame  and  the  included  lenses  may  -be 
tilted  to  any  desired  angle.  In  making  the  test  of  vision 
and  in  prescribing  spectacle  frames,  especially  for  near  work, 
this  is  frequently  of  great  advantage. 


THE  TRIAL  CASE  AND  ITS  USES.  53 


FIG.  35. — Showing  Trial  Frame  Properly  Adjusted  for  Conducting  a  Test. 


54 


REFRACTION. 


FIG.  36. — Showing  Authors'  Trial  Frame  Adjusted  for  Reading. 


CHAPTER  VI. 
HYPEROPIA  OR  FAR  SIGHT. 

INTRODUCTION. 

The  Emmetropic  eye  has  been  mentioned  as  one  which, 
when  at  rest,  /.  e.,  with  accommodation  relaxed,  focusses 
parallel  rays  in  a  sharp  image  upon  the  retina.  This  is  true, 
however,  only  of  central  rays  which  unite  in,  or  very  near 
to  the  fovea  centralis  or  macula  lutea,  that  point  on  the 
retina  affording  most  distinct  vision.  Peripheral  parallel 


FIG.  37. — Showing  the  Focus  of  Parallel  Rays  Behind  the  Retina  of  a 
Hyperopic  Eye. 

rays  do  not  converge  to  a  sharp  image  upon  the  retina,  even 
in  an  emmetropic  eye.  But  when,  in  the  absence  of  accom- 
modation, all  parallel  rays  fail  to  focus  on  the  retina  the 
condition  is  known  as  Ametropia. 

The  principal  forms  of  Ametropia  are  :  Hyperopia,  My- 
opia and  Astigmia. 

Hyperopia  is  also  known  as  Hypermetropia  or  Farsight- 
edness, expressed  by  the  letter  H.  In  this  form  of  Ametropia 


56 


REFRACTION. 


the  refractive  power  of  the  eye  is  insufficient.     Parallel  rays 
focus  behind  the  retina  and  only  by  the  aid  of  the  accommo- 


FIG.  38. — Showing  a  Hyperopic  Eye — Corrected  for  Distant  Vision. 

dation  can  they  be  made  to  focus  on  the  retina.  (Fig.  37.) 
The  divergent  rays  of  close  objects  focus  at  a  still  greater 
distance  back  of  the  retina.  In  this  form  of  Ametropia  a 


FIG.  39.—Showing  a  Hyperopic  Eye — Corrected  for  Near  Vision. 


convex  lens  is  required  to  bring  the  focus  forward  to  the 
retina.     (Fig.  38  and  Fig.  39.) 


HYPEROPIA   OR   FAR  SIGHT.  57 

Hyperopia  may  be  due  to  : 

(a)  Deficient  curvature   of  the  refracting  media,  called 
Curvature  Hyperopia.,  or 

(b)  Abnormal  shortness  of  the  eyeball,  called  Axial  Hy- 
peropia, or 

(c)  A  combination  of  both  (a)  and  (b). 

CAUSES. 

Hyperopia  is  usually  congenital.  By  the  growth  and  de- 
velopment of  the  eyeball,  it  may  be  changed  to  a  condition 
of  emmetropia,  or  more  rarely,  it  may  even  pass  into  my- 
opia. While  it  may  remain  as  a  latent  error,  it  becomes 
manifest  in  later  life. 

The  extraction  of  the  lens  for  cataract,  leaving  the  eye  in 
the  condition  known  as  Aphakia,  produces  one  form  of 
hyperopia. 

SYMPTOMS    AND    SEQUELAE. 

The  patient  finds  it  difficult  to  maintain  a  distinct  image 
of  small  objects,  and  consequently  complains  of  blurring 
vision.  He  is  forced  to  stop  reading  and  rub  his  eyes  ;  after 
this  his  sight  seems  better  for  a  moment,  but  very  soon  the 
haziness  of  vision  recurs.  The  accommodation  finally  be- 
comes exhausted  and  work  must  be  discontinued.  Children 
frequently  hold  their  books  near  to  the  face  and  partially 
close  the  eyes  to  cut  off  the  divergent  rays.  Too  often  this 
leads  to  an  erroneous  diagnosis  and  the  prescription  of  con- 
cave glasses.  Accommodation  and  convergence  are  such 
closely  related  acts,  nervously,  that  the  excessive  accommo- 
dation, used  in  high  degrees  of  hyperopia,  and  its  associated 
convergence  may  result  in  permanent  displacement  of  the 
eye,  a  condition  known  as  Strabismus. 

SPASM  OF  ACCOMMODATION. 

This  annoying  complication  is  due  to  the  persistent  con- 
traction of  the  ciliary  muscle  by  which  the  patient  actually 
makes  himself  myopic.  Distant  vision  is  rendered  indistinct 
and  the  patient  suffers  much  pain,  discomfort,  and  the  loss 
of  muscular  and  nervous  energy.  Spasm  is  apt  to  occur  in 
individuals  of  a  neurasthenic  type.  It  is  attended  by  con- 
5 


58  REFRACTION. 

junctivitis,  blepharitis  and  congestion  of  the  retina  and 
choroid.  Other  symptoms  are  headache,  aggravated  by 
using  the  eyes,  functional  nervous  disturbances  and,  some- 
times, convergent  squint.  While  concave  glasses  will  im- 
prove the  vision,  the  error  of  such  a  prescription  is  readily 
seen. 

VARIETIES  OF  HYPEROPIA. 

According  to  Bonders,  Hyperopia  is  Facultative  when  it 
can  be  overcome  by  accommodation  without  squinting; 
Relative  when  it  can  be  overcome  by  accommodation  only 
when  the  subject  squints;  and  Absolute  when  it  cannot  be 
overcome  by  any  efforts  of  the  accommodation. 

Manifest  hyperopia,  indicated  by  the  letters  H.  M.,  is  that 
amount  of  hyperopia,  which  becomes  apparent  without 
paralyzing  the  accommodation  by  means  of  a  mydriatic.  It 
is  measured  by  the  strongest  convex  lens  through  which  the 
eye  retains  distinct  vision. 

Latent  hyperopia,  indicated  by  the  symbols  H.  L.,is  that 
form  which  is  discovered  only  after  the  accommodation  has 
been  arrested  by  a  cycloplegic.  This,  when  added  to  the 
manifest,  is  the  total  hyperopia,  Hyperopia  Totalis,  indicated 
by  the  letters  H.  T. 

DETERMINATION  OF  HYPEROPIA. 

Hyperopia  always  exists : 

1.  When  distant  vision  is  not  reduced  by  a  convex  lens. 

2.  When  the  patient  can  read  fine  print  through  a  convex 
glass  at  a  greater  distance  than  its  focal  length. 

3.  And  usually  when  the  near  point  lies  at  a  greater  dis- 
tance from  the  eye  than  is  proper  for  the  age  of  the  patient. 

CORRECTION  OF  HYPEROPIA. 

With  the  patient  seated  six  meters  (twenty  feet)  from  the 
Snellen  test  card  and  wearing  a  properly  adjusted  test  frame, 
place  before  the  uncovered  eye  convex  glasses,  passing 
gradually  from  weaker  to  stronger  ones  until  the  best  vision 
has  been  obtained.  The  strongest  convex  lens  giving  this 
result  is  the  measure  of  the  manifest  hyperopia  (H.  M.). 


HYPEROPIA   OR   FAR  SIGHT.  59 

It  will  be  found  frequently  that  a  hyperopic  person,  be- 
cause his  accommodation  overcomes  the  error,  has  perfect 
vision  with  the  unaided  eye.  In  such  a  case,  of  course,  it  is 
impossible  to  improve  the  vision  by  means  of  convex  lenses, 
indeed,  concave  lenses  may  appear  to  afford  some  improve- 
ment. Such  an  eye  does  its  work  satisfactorily  as  to  vision, 
but  at  too  great  a  cost  in  energy ;  glasses  will  enable  it  to 
perform  its  function  with  a  much  smaller  expenditure  of  mus- 
cular effort.  In  such  a  case  the  manifest  hyperopia  is  meas- 
ured by  the  strongest  convex  lens  with  which  the  patient  can 
see  as  well  as  he  does  with  the  unaided  eye.  The  very  fact 
that  he  sees  as  well  at  a  distance  with  a  convex  glass  as 
without  it,  proves  that  the  eye  is  hyperopic ;  the  emmetropic 
eye  and,  to  a  greater  degree,  the  myopic  eye  sees  less  dis- 
tinctly through  even  a  weak  convex  lens. 

In  testing  the  eye  for  distant  vision  by  means  of  the  Snel- 
len  types  at  a  range  of  six  meters,  it  is  essential  that  the  ac- 
commodation be  relaxed.  It  is  desirable  throughout  the 
test  to  disturb  the  natural  conditions  as  little  as  possible. 
While  it  is  the  practice  in  most  public  clinics  to  employ  in 
every  case  a  mydriatic,  as  a  matter  of  routine  and  to 
economize  time,  such  procedure  in  private  practice  is  neces- 
sary only  in  exceptional  cases.  The  refractionist  who  is 
skilled  in  the  use  of  the  test  case  and  the  confirmatory  tests, 
and  who  has  tact  and  patience,  may  learn  to  coax  out  the 
most  obstinate  accommodation  and  accurately  measure  the 
degree  of  eye  strain  without  the  routine  use  of  a  mydriatic. 
The  more  skillful  the  refractionist,  the  less  will  he  depend 
upon  the  drug. 

In  order  to  relax  the  ciliary  muscle,  there  are  some  meas- 
ures which  will  be  found  of  value.  The  following  may  be 
employed : 

1.  Place  before  one  eye,  its  fellow  being  covered,  convex 
lenses  of  increasing  strength  until  the  image  is  blurred,  then 
gradually   weaken  to  a  point   of  distinct   vision.     In  other 
words,  overcorrect  the  error  and  then  return  to  a  lens  which 
gives  clear  and  distinct  vision. 

2.  Place  before  each  eye  a  strong  convex  lens,  for  instance, 
a  plus  4  D.  S.,  allowing  the  patient  to  wear  them  ten   or 


60  REFRACTION. 

fifteen  minutes,  then  gradually  neutralize  by  superimposing 
concave  lenses  of  increasing  strength  to  the  point  of  best 
vision.  . 

3.  Sometimes  a  two-degree  prism,  base  in,  will  serve  to 
relax  the  Internal  Rectus  muscle  and  the  accommodation. 

In  every  case,  the  lenses  should  be  increased  in  strength 
very  gradually,  the  accommodation  relaxing  by  this  method. 
Great  deliberation  is  of  positive  value.  It  is  absolutely  im- 
portant to  quiet  the  nervousness  of  an  excitable  patient; 
in  every  case,  to  win  the  confidence  of  the  patient  makes  the 
task  much  easier. 

In  myopes  and  in  the  majority  of  hyperopic  and  astigmic 
patients,  the  above  measures  will  be  sufficient.  If,  however, 
the  statements  of  the  patient  are  hesitating,  varying  and 
inconsistent,  no  definite  conclusions  can  be  reached  by  the 
examiner;  in  such  a  case  a  cycloplegic  must  be  employed. 

CYCLOPLEGICS  AND    MYDRIATICS. 

In  view  of  the  fact  that  paralysis  of  accommodation  is 
usually  accompanied  by  a  simultaneous  dilatation  of  the 
pupil,  the  terms  cycloplegic  and  mydriatic  are  ordinarily  used 
synonymously.  The  majority  ofcycloplegics  are  also  mydriat- 
ics.  True  mydriatics  are  those  which  dilate  the  pupil  with- 
out affecting  the  accommodation,  e.g.,  Euphthalmin,  Ephed- 
rin,  Mydrin  and  Cocain.  The  most  commonly  used  cyclo- 
plegics  are  Atropin  Sulphate,  Homatropin  Hydrobromate, 
Scopolamin  and  Duboisin. 

In  choosing  between  cycloplegics,  the  chief  factor  to  be 
considered  is  the  age  of  the  patient.  Atropin  (in  1  per  cent, 
solution  usually),  being  the  strongest,  is  best  suited  to  the 
more  active  accommodation  of  children  and  young  adults; 
Homatropin  (in  1  per  cent,  solution),  alone,  or  in  combina- 
tion with  Cocain  (2  per  cent,  solution),  being  employed  to 
combat  the  less  active  accommodation  of  patients  from 
twenty-five  to  thirty  years  of  age.  After  the  age  of  thirty- 
five,  spasm  of  accommodation  is  very  rare  and  hence  the 
use  of  a  cycloplegic  is  rarely  required. 

Contraindications  for  the  Use  of  Mydriatics. — In  any  case 
of  increased  tension  of  the  eyeball  at  any  age,  the  use  of 


HYPEROPIA   OR   FAR  SIGHT. 


61 


a  mydriatic  is  interdicted.  This  includes  suspected  cases  of 
glaucoma  as  well  as  those  in  which  the  disease  is  actually 
present. 

TABLE    III. 

SHOWING  STRENGTH  OF  CYCI/)PI,EGICS  AND    MYDRIATICS,  TIME    REQUIRED 
TO  ACT  AND  DURATION  OF  DRUG  EFFECT. 


Drug. 

Strength  Employed. 

Time      Required     t  o 
Reach  Maximum  Ef- 
fect. 

Duration     of    Drug 
Action. 

Atropin      Sul- 
phate. 

I  %  solution. 

48  hours. 

7  to  10  days. 

H  omat  ropi  n 
Hydrobromate 
(several  instal- 
lations   at    in- 

I %  solution. 

2  hours. 

I  to  2  days. 

tervals   of   ten 

minutes). 

Scopolamin  Hy- 
drobromate. 

i-io  %  solution. 

i  hour. 

4  days. 

Duboisin      Sul- 
phate. 

%  %  solution. 

Yz  hour. 

4  days. 

Euphthalmin 
Hydro  chlor- 

5-10 %  solution. 

20  minutes. 

5  hours. 

ate. 

Ephedrin  H  y  - 

10  %  solution. 

40  to  60  minutes. 

5  to  20  hours. 

drochlorate. 


Cocain    Hydro- 
chlorate. 

2  %  solution. 

I  hour. 

3  to  4  hours. 

Correction  of  Results  Obtained  Under  a  Cycloplegic. — 
That  lens  or  combination  of  lenses  which  gives  the  patient 
the  best  distant  vision  while  under  the  influence  of  a  cyclo- 
plegic  expresses  the  total  error  of  refraction,  i.  e.,  the  mani- 
fest plus  the  latent  error.  The  latter  was  brought  out  by 
the  cycloplegic  and  represents  thetonicity  of  the  ciliary  mus- 
cle. To  prescribe  the  full  correction,  i.  e.,  the  strength  of 
lens  giving  most  perfect  vision  under  mydriasis,  would  mani- 
festly be  a  grave  error,  because : 


62  REFRACTION. 

1.  The  ciliary  muscle,  which  in  hyperopic  eyes  is  more  or 
less  hypertrophied,  without  drug  effect  is  unable  to  fully  re- 
lax ;  therefore,  the  accommodation  is  never  fully  relaxed  in 
the  hyperopic  eye. 

2.  If  the  ciliary  muscle  could  be  fully  relaxed,  to  keep  it  so 
by  prescribing  glasses  to  perform  the  functions  normally  its 
province,  would  not  only  cause  the  patient  great  discomfort, 
but  would  result  sooner  or  later  in  muscular  atrophy  from 
disuse. 

For  this  reason,  it  is  necessary  to  deduct  from  the  meas- 
urement under  atropin  or  other  cycloplegic,  from  1 D.  to  1.50 
D.  in  the  case  of  children  and  young  adults,  and  from  .75  D. 
to  1  D.  in  older  individuals.  For  example,  if  the  total  error 
of  refraction,  as  revealed  by  the  cycloplegic,  is  +  3  D.  S.,  the 
prescription  for  glasses  would  be  from  +  1.50  D.  S.  to  +  2 
D.  S.  in  a  child,  and  from  +  2  D.  S.  to  +  2.25  D.  S.  for  an 
adult. 

MYOTICS. 

Myotics  are  drugs  which  produce  a  contraction  of  the 
pupil  and  may  induce  spasm  of  accommodation.  Those 
generally  emplo3*ed  in  medical  practice  are  Eserin  (in  1  per 
cent,  solution)  and  Pilocarpin  (in  Vi  and  y%  per  cent,  solu- 
tion). They  are  used  to  counteract  the  effect  of  mydriatics 
and  to  draw  the  iris  away  from  the  iris  angle  in  glaucoma 
and  in  peripheral  lesions  of  the  cornea. 

After  using  a  cycloplegic  like  homatropin,  its  annoying 
effect  may  be  temporarily  overcome  by  the  use  of  a  myotic. 
Frequent  installations  of  eserin,  for  instance,  will  counteract 
the  action  of  the  cycloplegic. 


CHAPTER  VII. 
MYOPIA  OR  NEAR  SIGHT. 

INTRODUCTION. 

As  the  correction  of  hyperopia  necessitates  an  increase  of 
the  refractive  power  of  the  eye  by  means  of  convex  glasses, 
so  that  of  Myopia  demands  a  decrease  of  the  refractivity,  ac- 
complished by  the  use  of  concave  lenses.  In  Myopia  the  re- 
fractivity is  too  great  for  the  length  of  the  eyeball,  so  that 
parallel  rays  focus  in  front  of  the  retina.  (Fig.  4-0.)  In  this 


FIG.  40. — Showing  the  Focus  of  Parallel  Rays  in  Front  of  the  Retina  of  a 

Myopic  Eye. 

condition  the  only  rays  falling  on  the  retina  emanate  from 
objects  nearer  the  eye  than  infinity.  The  more  the  rays  di- 
verge, i.  e.,  the  nearer  the  object  viewed  is  to  the  eye,  the 
more  distinct  will  be  the  vision.  Hence,  in  myopia  of  mild 
degree,  the  patient  requires  correcting  lenses  for  his  distant 


64 


REFRACTION. 


vision  only,  no  glasses  being  needed  for  close  work.  (Fig.  41.) 
In  nvyopia  of  high  degree,  the  glasses  prescribed  for  distant 
vision  are  too  strong  for  reading.  A  second  pair  of  glasses 
of  much  weaker  power  are,  therefore,  required  for  close 
work. 

Myopia  may  be  due  to : 

(a)  Increased    length    of  the    eyeball,   known   as  Axial 
Myopia,  or 

(b)  Increased  curvature  of  the  refracting  media,  known 
as  Curvature  Myopia,  or 


FIG.  41.— Showing  Divergent  Rays  of  a  Near  Object  Focussed 
on  the  Retina  of  a  Myopic  Eye. 

(c)   A  combination  of  both  these  conditions. 

The  great  majority  of  cases  of  Myopia  are  probably 
axial,  those  cases  due  to  increased  curvature  of  the  refract- 
ing media  being  much  less  frequent.  Curvature  Myopia  may 
be  of  two  kinds: 

(a)  That  affecting  the  cornea,  and 

(b)  That  affecting  the  lens. 

As  an  example  of  the  former  may  be  mentioned  the  condi- 
tion known  as  Keratoconus  or  Bulging  Cornea,  while  the 
latter  is  well  illustrated  by  the  swelling  of  the  lens  which 


MYOPIA   OR   NEAR   SIGHT.  65 

sometimes  takes  place  in  incipient  cataract.  As  a  result  of 
the  increased  refractivity  caused  b}^  this  swelling,  the  patient 
is  able  to  read  without  the  aid  of  glasses,  a  condition 
known  to  the  laity  as  "  second  sight." 

CAUSES  OF    MYOPIA. 

% 

Myopia  is  very  largely  the  result  of  the  unreasonable  de- 
mands which  civilization  and  higher  education  make  upon 
the  eyes.  The  excessive  and  improper  use  of  the  eyes 
for  near  work  during  the  period  of  development  seems 
to  be  the  chief  predisposing  cause.  Heredity  is  also  an 
important  causative  factor.  The  exciting  cause  is  usually 
found  in  improper  hygienic  conditions,  e.g.,  poor  ventilation 
and  insufficient  illumination  of  school  rooms,  faulty  position 
of  the  head  and  body  during  study  and  sedentary  habits. 

The  lengthening  of  the  eyeball  found  in  axial  myopia  may 
be  brought  about  in  two  ways: 

(1)  Individuals,  whose  eyes  are  far  apart,  are  compelled 
to  exercise  an  excessive  amount  of  convergence  when  fixing 
upon  near  objects.    By  this  excessive  convergence  the  extra- 
ocular  muscles  exert  undue  pressure  on  the  eyeballs,  causing 
a  bulging  of  the  less  resistent  posterior  pole,  thereby  pro- 
ducing the  condition  known  as  Posterior  Staphyloma. 

(2)  As  a  result  of  chronic  inflammatory  processes  and 
the  destructive  changes  caused  thereby,  the  coats  of  the  eye- 
ball become  softened  and  less  resistent  and  finally  yield  to 
intra-ocular  and    extra-ocular   (muscular)    pressure.     This 
form   of  myopia  is  usually  progressive,  often  attaining  a 
high  degree  and  sometimes  resulting  in  total  blindness. 

SYMPTOMS   OF  MYOPIA. 

The  patient's  distant  vision  is  poor.  The  eyes  are  sensitive 
to  light  and  tire  easily,  becoming  painful  after  near  work. 
Patients  frequently  complain  of  black  spots  before  the  eyes 
and  occasionally  of  flashes  of  light. 

In  myopia  of  high  degree  the  eyeballs  are  often  prominent 
and  staring,  giving  the  face  a  rather  stupid  expression.  The 
patient,  when  reading,  moves  his  whole  head  instead  of  his 
eyes  alone,  thus  following  the  lines  back  and  forth  across 


66 


REFRACTION. 


the  page.  The  ophthalmoscope  shows  the  optic  disc  to  ap- 
pear enlarged.  There  may  be  divergent  squint,  the  internal 
recti  being  unequal  to  the  task  of  maintaining  the  ex- 
cessive amount  of  convergence  necessary. 

DETERMINATION   OF   MYOPIA. 

• 

Myopia  always  exists  when,  in  the  absence  of  spasm  of 
the  ciliary  muscle : 

1st.  Distant  vision  is  improved  by  concave  lenses  and 
blurred  by  cqnvex  lenses ; 

2d.  The  far  point  (punctum  remotum)  lies  nearer  the  eye 
than  infinity. 


FIG.  42. — Showing  a  Myopic  Eye  Corrected  for  Distant  Vision. 
CORRECTION   OF   MYOPIA. 

With  the  patient  seated  twentv-  feet  from  the  test  card, 
place  before  the  eye,  testing  each  eye  separately,  concave 
lenses  of  different  strength  until  the  most  distinct  vision  is 
obtained.  The  weakest  lens  giving  this  result  is  the  measure 
of  the  mvopia  and  should  be  prescribed  for  distant  use. 
(Fig.  42.)" 

Should  the  error  be  one  of  high  degree,   e.  g.,   from   5   D. 
to  8    D.,  or  the  accommodation  feeble,  a  weaker  lens  af- 


MYOPIA   OR   NEAR  SIGHT.  67 

fording  good  vision  at  14  to  16  inches  should  be  prescribed 
for  reading.  This  is  a  point  deserving  special  emphasis,  be- 
cause it  is  too  frequent-lv-  disregarded. 

PROPHYLAXIS. 

A  myope  should  be  instructed  in  a  proper  hygienic 
regime,  e.  g.,  the  use  only  of  books  having  large  distinct 
type,  the  best  possible  light,  proper  ventilation  of  the  study 
or  school  room,  plenty  of  out-door  exercise  and  the  limita- 
tion of  close  work  to  the  minimum.  He  should  be  instructed 
to  wear  his  glasses  only  for  distant  vision,  or  if  two  correc- 
tions are  prescribed,  the  weaker  one  for  reading  should  be 
the  invariable  practice. 


CHAPTER  VIII. 
ASTIGMIA. 

INTRODUCTION. 

A  beam  of  light  on  passing  through  a  convex  cylindrical 
lens  becomes  wedge-shaped,  the  sharp  edge  of  the  wedge 
being  the  focal  line  (See  p.  21).  The  base  of  the  wedge  lies 
against  the  lens.  (See  Fig.  14.)  By  placing  together  two  con- 
vex cylindrical  lenses  of  different  refractive  strength,  with 
their  axes  at  right  angles  to  each  other,  there  will  be  formed 


FIG.  43. — Showing  the  Effect  of  Crossed  Cylinders  as  Illustrating  the 
Refractive  Effect  of  an  Astigmic  Surface. 

two  wedges,  a  short  one  by  the  stronger  lens  and  a  longer 
one  by  the  weaker  lens.  The  edges  or  focal  lines  will  lie  some 
distance  apart  and  at  right  angles  to  each  other.  (Fig. 43.) 
The  same  result  is  obtained  by  combining  a  spherical  lens 
with  a  cylinder  (See  Relationship  of  Lenses). 

This  represents  the  refractive  condition   of  some  eyes. 
Either  the  cornea,  or  the  crystalline  lens,  or  both,  so  change 


ASTIGMIA.  69 

the  transmitted  light  rays  as  to  have  the  effect  of  a  sphero- 
cylindrical  lens.  The  result  is  that,  instead  of  focussing  the 
rays  of  light  at  one  point  upon  the  retina,  two  focal  lines 
are  formed  some  distance  apart  and  at  right  angles  to  each 
other.  One  of  these  lines  may  be  upon  the  retina  or  both 
may  be  behind  or  in  front  of  that  structure.  An  eye  with 
this  peculiar  refractive  power  is  said  to  be  astigmic,  and 
the  condition  is  known  as  Astigmia  or  Astigmatism.  The 
cause  of  astigmia  usually  lies  in  the  cornea,  which,  instead 
of  being  like  a  section  of  a  sphere,  resembles  more  the  bowl 
of  a  spoon,  /.  e.,  one  meridian  is  more  convex  than  that 
which  is  at  right  angles  to  it,  having,  therefore,  a  shorter 
focal  distance. 

In  view  of  the  fact  that  the  majority  of  astigmic  eyes 
show  a  more  pronounced  curvature  vertically  than  hori- 
zontally, astigmia  at  this  angle  is  said  to  be  "according  to 
the  rule."  The  opposite  condition,  viz.,  that  in  which  the 
horizontal  meridian  shows  the  greater  curvature,  is  spoken 
of  as  astigmia  "  against  the  rule." 

In  a  general  way  Astigmia  may  be  divided  into  two 
classes,  as  follows : 

Regular  Astigmia  is  that  form  in  which  all  the  meridians 
of  the  refracting  surfaces  of  the  eye  are  regular  in  curvature, 
i.  e.,  they  are  the  arcs  of  circles.  This  is  the  condition  which 
especially  concerns  the  refractionist  because  it  can  be  cor- 
rected by  the  use  of  glasses. 

Irregular  Astigmia,  of  interest  to  the  ophthalmologist, 
but  hopeless,  optically  speaking,  is  caused  by  a  deformity  of 
the  cornea  resulting  from  a  burn,  injury,  ulcer  or  other  dis- 
eased condition,  followed  by  the  formation  of  scar  tissue. 
The  latter,  by  its  contraction,  produces  an  irregular,  refract- 
ing surface,  the  effect  of  -which  is  to  so  distort  the  image 
formed  on  the  retina  that  no  lens,  or  combination  of  lenses, 
will  serve  to  bring  the  different  parts  of  the  image  into  one 
plane.  It  is  rarely  improved  by  the  use  of  glasses. 

FORMS    OF    REGULAR    ASTIGMIA. 

There  are  five  principal  forms  of  regular  astigmia,  viz.: 
(1)    Simple  Hyperopic,  in  which  one  meridian  focusses  on 
the  retina,  the  opposite  meridian  behind  it. 


70 


REFRACTION. 


(2)   Simple  Myopic,  in  which  one  meridian  focusses  on  the 
retina,  the  opposite  one  in  front  of  it. 


FIG.   44. — Showing    Varieties    of    Astigmia — I.    Emmetropia;    2.    Simple 

Hyperopic  Astigmia;  3.  Simple  Myopic  Astigmia;  4.  Compound 

Hyperopic  Astigmia;  5.  Compound  Myopic  Astigmia; 

6.  Mixed  Astigmia. 


(3)    Compound  Hyperopic,  in  which  both  meridians  focus 
behind  the  retina,  but  at  different  distances  from  it. 


ASTIGMIA.  7 1 

(4)  Compound  Myopic,  in  which  both  meridians  focus  in 
front  of  the  retina,  but  at  different  distances  from  it. 

(5)  Mixed,  in  which  one  meridian  focusses  behind  and  the 
other  in  front  of  the  retina. 

The  different  forms  are  illustrated  by  Fig.  44.  (Also  by 
Fig.  43,  the  retina  being  interposed  at  the  points  indicated.) 

CAUSES  OF  REGULAR  ASTIGMIA. 

Astigmia  is  usually  congenital  and  frequently  associated 
with  other  defects  of  vision.  Ordinarily  it  is  found  to  affect 
both  eyes,  usually  to  the  same  degree  and  in  the  same  me- 
ridian. When  acquired,  it  is  usually  caused  by  pressure  upon 
the  eyeball  of  the  lids  or  external  eye  muscles,  or  by  opera- 
tion, e.g.,  cataract  extraction  or  iridectomy.  Astigmia  may 
also  be  caused  by  an  oblique  position  of  the  lens  of  the  eye, 
due  to  accidental  or  natural  subluxation.  (See  Relation- 
ship of  Lenses.) 

SYMPTOMS. 

These  are  the  usual  symptoms  of  Asthenopia.  When  the 
error  is  comparatively  slight,  i.  e.,  below  1  D.,  no  indistinct- 
ness of  vision  is  noticed.  When  more  than  1  D.,  however, 
circular  bodies  will  appear  elliptical  and  straight  lines  fall- 
ing in  the  faulty  meridians  will  appear  indistinct.  The  pa- 
tient makes  frequent  mistakes  in  reading  the  test  types  at 
twenty  feet.  Astigmia  should  always  be  suspected  if  the  pa- 
tient in  attempting  to  read  the  letters  on  the  test  card, 
calls  some  correctly  and  makes  guesses  as  to  others.  There  is 
more  or  less  blurring  of  the  letters  when  reading  at  close 
range.  It  is  not  unusual  to  find  assymetry  of  the  face  asso- 
ciated with  astigmia,  the  eye  apparently  sharing  the  facial 
irregularity. 

DETERMINATION   OF  ASTIGMIA. 

The  examination  proceeds  exactly  as  in  the  determination 
of  Hyperopia  or  Myopia.  The  patient  is  given  the  best 
vision  possible  by  the  use  of  spherical  lenses.  When  it  is  im- 
possible to  render  vision  normal  by  the  use  of  spherical 
lenses,  it  is  probably  owing  to  the  presence  of  astigmia. 


72 


REFRACTION. 


A  simple  way  of  detecting  astigmia  is  by  rotating  before 
the  eye  a  weak  cylinder,  say  a  plus  .50  D.  If  the  vision  is 
made  worse  in  all  meridians  no  appreciable  amount  of  as- 
tigmia is  present.  If  the  vision  is  better  at  any  one  meridian, 


90 


50  Feet 


P5T2E6R 

30  Feet 

8F4O9G3T5 

20  Feet 

D2R6A5Z8P9H4E 

3N9V4T8PZ1B5K3R6D2 

E5C8L2R4Y3T6etA2N4Z5P3V8R 

FIG.  45. — Showing  Fan  of  Rays. 

astigmia  is  present  and  the  axis  of  the  cylinder  giving  most 
distinct  vision  is  at  right  angles  to  the  faulty  meridian. 

A  very  useful  aid  in  determining  astigmia  is  the  fan  of 
rays  (Fig.  45)  or  the  astigmic  dial  (Fig.  46.)     This  is  used  at 


ASTIGMIA. 


73 


a  distance  of  twenty  feet  and  all  the  tests  are  made  with  one 
eye  at  a  time.  When  astigmia  is  manifest,  i.  e.,  when  it  is 
not  concealed  by  the  patient's  accommodation,  one  or  two 
rays  of  the  fan  or  radiating  lines  of  the  dial  appear  more  dis- 
tinct than  the  remainder  and  indicate  that  the  normal 
meridian  lies  at  right  angles  to  these.  (Fig.  47.)  When  the 


FIG.  46. — Showing  Astigmic  Dial. 

astigmia  is  latent,  i.  e.,  concealed  by  the  patient's  accommo- 
dation, it  can  usually  be  made  manifest  by  revolving  cylin- 
ders of  varying  degrees  of  strength  before  the  eye.  The  cylinder 
at  some  axis  will  cause  one  or  two  of  the  rays  or  lines  to  ap- 
pear blacker.  This  axis  coincides  with  the  faulty  meridian. 
6 


74  REFRACTION. 

When  this  method  fails  a  cycloplegic  must  be  employed  to 
put  the  patient's  accommodation  entirely  at  rest. 

CORRECTION   OF  ASTIGMIA. 

General  Remarks. — In  the  correction  of  astigmia,  just  as 
in  the  neutralization  of  a  cylindric  lens  (see  Neutralization  of 


FIG.   47. — Showing  Dial  a*   Seen   by   an    Astigmic   Eye. 

Lenses),  three  things  must  be  ascertained  regarding  the  cor- 
recting lens,  viz.,  the  kind,  the  strength  and  its  position  be- 
fore the  eye,  i.  e.,  the  axis.  The  latter  is  indicated  in  degrees 
of  an  arc  numbering  from  right  to  left.  (See  Fig.  16.)  By 
placing  at  the  proper  axis  before  the  eye  convex  or  concave 


ASTIGMIA. 


75 


cylinders  of  varying  degrees  of  strength,  the  strongest  con- 
vex, or  the  \veakest  concave,  cylinder  through  -which  vision 
is  clearest  and  all  of  the  rays  or  lines  are  seen  with  equal  dis- 
tinctness, is  the  measure  of  the  astigmia. 

Simple  Hyperopic  Astigmia  (H.  As.). — By  placing  before 
the  eye  a  convex  cylinder,  of  the  right  strength  and  at  the 
proper  axis,  the  rays  which  formed  a  focal  line  behind  the 
retina  are  converged  to  meet  on  the  retina.  (Fig.  48.)  The 
strongest  convex  cylinder  which  makes  the  patient's  vision 


FIG.  48. — Showing  Position  of  Focal  Lines  in  Simple  Hyperopic  Astigmia 
and  Position  of  Correcting  Cylindric  Lens. 

clear  and  all  the  lines  on  the  dial  appear  equally  distinct  is 
the  correction  of  the  condition  and  should  be  prescribed  for 
constant  wear.  When  the  lens  prescribed  is  weak  and 
the  patient's  symptoms  slight,  an  exception  may  be  made 
and  the  patient  be  permitted  to  wear  his  glasses  for  near 
work  only. 

In  some  cases  in  which  the  error  is  one  of  very  high  degree, 
the  full  correction  cannot  be  prescribed,  owing  to  the  great 
discomfort  experienced  by  the  patient.  It  then  becomes 


76 


REFRACTION. 


necessary  to  reduce  the  strength  of  the  lens  sufficiently  to  in- 
sure comfort,  and,  at  the  same  time,  render  vision  as  good 
as  possible.  The  final  prescription  in  such  a  case  can  be  as- 
certained only  by  repeated  trials. 

Simple  Myopic  Astigmia  (M.  As.). — By  placing  before  the 
eye  a  concave  cylinder,  of  the  right  strength  and  at  the 
proper  axis,  the  rays  which  formed  a  focal  line  in  front  of  the 
retina,  Fig.  49,  are  made  less  convergent  so  that  they  meet 


FIG.  49. — Showing  Position  of  Focal  Lines  in  Simple  Myopic  Astigmia  and 
Position  of  Correcting  Cylindric  Lens. 

on  the  retina.  The  weakest  concave  cylinder  which  makes 
the  patient's  vision  clear  and  all  the  lines  on  the  dial  appear 
equally  distinct  is  the  correction  of  the  condition. 

In  cases  of  low  degree,  this  correcting  cylinder  may  be 
prescribed  for  constant  wear.  When  the  strength  of  the 
lens  prescribed  exceeds  .50  to  .75  D.  a  second  pair  must  be 
prescribed  for  near  work.  Usually  it  is  sufficient  to  prescribe 


ASTIGMIA. 


77 


convex  cylinders  of  the  same  strength  at  the  opposite  axis 
for  near  work.  In  this  way  the  emmetropic  meridian  is 
brought  forward  to  the  same  plane  with  the  faulty  meridian, 
thus  changing  the  condition  to  one  of  myopia,  which  the  pa- 
tient will  disregard  for  reading,  if  the  error  be  of  low  degree. 

Example.  For  distant  vision :  —  .75  D.  C.  Ax.  180°  ;  for 
reading:  +  .75  D.  C.  Ax.  90°. 

When  the  error  is  one  of  high  degree,  e.  g.,  4  D.  to  6  D.,  it 
is  frequently  necessary  to  prescribe,  for  reading,  a  crossed 
cylindrical  lens,  i.  e.,  a  concave  cylinder  to  carry  the  faulty 
meridian  toward  the  retina  and  a  convex  cylinder  at  right 


FIG.  50. — Showing  Position  of  Focal  Lines  in  Compound  Hyperopic  Astig- 
mia  and  Position  of  Correcting  Cylindric  Lenses. 

angles  to  the  above  to  carry  the  emmetropic  meridian  for- 
ward to  the  same  plane,  thus  again  producing  a  condition  of 
myopia. 

Example.  For  distant  vision:  --6  D.  C.  Ax.  75°;  for 
reading-  3  D.  C.  Ax.  75°  C  +  3  D.  C.  Ax.  165°.  This  pre- 
scription may  also  be  written:  —  6  D.  C.  Ax.  75°  +  3  D.  S. 
(See  Relationship,  Page  33.) 

Compound  Hyperopic  Astigmia  (C.  H.  As.). — In  this  con- 
dition both  focal  lines  are  behind  the  retina,  but  at  different 
distances.  (Fig.  50.)  Ry  placing  a  convex  spherical  lens  of 


78 


REFRACTION. 


the  proper  strength  before  the  eye, both  focal  lines  are  moved 
forward,  one  falling  on  the  retina,  the  other  still  behind  the 
retina,  but  nearer  than  before.  Placing  a  convex  cylin- 
dric  lens  of  the  right  strength  over  the  spherical  lens  at  the 
proper  axis,  the  rays  which  formed  the  focal  line  behind  the 
retina  are  converged  to  meet  in  a  point  on  the  retina.  The 
strongest  convex  spherical  and  cylindrical  lenses,  which  com- 
bined make  the  patient's  vision  clear  and  all  the  lines  on  the 
dial  to  appear  equally  distinct,  are  the  correction  for  the 
condition. 


FIG.  51. — Showing  Position  of  Focal  Lines  in  Compound  Myopic  Astigmia 
and  Position  of  Correcting  Cylindric  Lenses. 

When  the  error  is  of  low  degree,  the  glasses  prescribed 
may  be  worn  for  close  work  only.  In  the  majority  of  cases, 
however,  they  should  be  prescribed  for  constant  wear. 

Compound  Myopic  Astigmia  (C.  M.  As.).— In  this  con- 
dition both  focal  lines  are  in  front  of  the  retina,  but  at  differ- 
ent distances  from  it.  (Fig.  51.)  By  placing  a  concave  spheri- 


ASTIGMIA.  79 

cal  lens  of  the  proper  strength  before  the  eye,  both  focal  lines 
are  moved  back,  one  falling  on  the  retina,  the  other  still 
in  front  of  it,  but  nearer  than  before.  Placing  over  the 
spherical  lens  a  concave  cylindric  lens,  of  the  right  strength 
and  at  the  proper  axis,  the  rays  which  formed  the  focal  line 
in  front  of  the  retina  are  made  less  convergent,  so  that  they 
meet  in  a  point  on  the  retina.  The  weakest  concave  spheri- 
cal and  cylindrical  lenses,  \vhich  combined  make  the  patient's 
vision  clear  and  all  the  lines  in  the  dial  to  appear  equally  dis- 
tinct, are  the  correction  for  the  condition. 

When  the  error  is  of  low  degree,  one  prescription  may  be 
sufficient,  the  patient  being  instructed  to  wear  the  glasses 
constantly.  When  moderately  strong  lenses  are  prescribed 
for  distance,  the  spherical  part  may  be  dropped  and  convex 
cylinders  be  ordered  for  reading. 

Example.  For  distant  vision:  -  1  D'.  S.  C  ~"  1.25  D. 
C.  Ax.  45°;  for  reading:  -  1.25  D.  C.  Ax.  135°. 

When  the  error  is  one  of  high  degree : 

(a)  The  same  cylinder  and  axis  may  be  prescribed  for 
reading,  or, 

(b)  The  same  cylinder  and  axis,  combined  with  a  weaker 
spherical  lens. 

Examples,  (a)  For  distant  vision:  —  3  D.  S.  O  ~  4  D. 
C.  Ax.  15°;  for  reading:  —  4  D.  C.  Ax.  15°. 

(b)  For  distant  vision:  -  6  D.  S.  C  -  3  D.  C.  Ax.  15°; 
for  reading:  -  2  D.  S.  C  -  3  D.  C.  Ax.  15°. 

Mixed  Astigmia  (M.  A.). — When  it  is  found  impossible  by 
means  of  spherical  or  cylindrical  lenses,  or  combinations  of 
the  same,  to  secure  distinct  vision,  mixed  astigmia  is  prob- 
ably present.  Cases  which  at  first  appear  to  be  high  degrees 
of  myopic  or  Iryperopic  astigmia,  on  careful  examination, 
may  be  found  to  be  mixed. 

In  this  condition,  one  focal  line  lies  behind  the  retina,  the 
other  in  front  of  the  retina.  (Fig.  52.)  By  placing  before  the 
eye  a  convex  cylinder,  of  the  right  strength  and  at  the  proper 
axis,  the  rays  which  formed  the  focal  line  behind  the  retina 
are  made  more  convergent  so  that  they  meet  in  a  line  on  the 
retina.  Placing  before  the  eye  a  concave  cylinder,  of  the 
right  strength  and  at  the  proper  axis  (at  right  angles  to  the 


80  REFRACTION. 

convex  cylinder),  the  rays  which  formed  a  focal  line  in  front 
of  the  retina  are  made  less  convergent  so  that  they  meet  on 
the  retina. 

Glasses  prescribed  for  Mixed  Astigmia  should  be  worn 
constantly.  When  the  myopic  cylinder  is  of  high  degree,  a 
separate  correction  should  be  made  for  reading,  in  which 
case  the  concave  cylinder  is  decreased  and  the  convex  cylin- 
der correspondingly  increased. 


FIG.  52. — Showing   Position  of  the   Focal  Lines  in   Mixed  Astigmia   and 
Position  of  Correcting  Cylindric  Lenses. 

Example.  -  3.50  D.  C.  Ax.  180°  C  +  1-75  D.  C.  Ax.  90°. 
For  reading:  -  1.50  D.  C.  Ax.  180°  C  +  3.75  D.  C.  Ax.  90°. 
Stenopseic  Disc.  —  Another  method  of  determining  the 
presence  of  astigmia  is  by  revolving  before  the  eye  the  steno- 
paeic  disc.  (See  p.  51.)  When  in  the  position  affording  most 
distinct  vision,  place  before  the  slit  convex  and  concave 
spherical  lenses;  the  strongest  convex  or  the  weakest  con- 
cave lens,  giving  the  greatest  improvement,  represents  the 
measure  of  the  refraction  in  this  meridian.  The  slit  is  then 


ASTIGMIA.  81 

revolved  to  a  position  at  right  angles  to  its  former  axis  and 
the  refraction  of  this  meridian  ascertained  in  the  same  man- 
ner. The  sum  of  the  two  results  obtained  is  the  total  error 
of  refraction. 

Example.     Vertical  meridian  :    f  .25  D.  S. 

Horizontal  meridian :   +  .75  D.  S. 

Total  error:    -r  25  D.  S.  C  +  .50  Ax.  90°. 


CHAPTER  IX. 

ANISOMETROPIA,  APHAKIA,  AMBLYOPIA  AND 
MALINGERING. 

ANISOMETROPIA. 

By  the  term,  Anisometropia,  is  meant  a  difference  in  the 
retractive  power  of  the  two  eyes.  One  eye  may  be  normal, 
or  both  hyperopic,  myopic  or  astigmic,  one  more  so  than 
the  other.  When  the  difference  is  slight,  both  eyes  may  be 
corrected.  When  the  difference  is  pronounced,  the  glass  re- 
quired to  correct  the  poorer  eye  is  often  too  strong  to  be 
worn  with  comfort,  causing  diplopia  or  producing  other  un- 
comfortable symptoms.  Therefore,  it  is  frequently  necessary 
to  prescribe  a  partial  correction  or  even  none  at  all  for  the 
poorer  eye,  and  complete  correction  for  the  better  eye. 

APHAKIA. 

After  the  extraction  of  acataract,  the  patient  is  compelled 
to  wear  two  pairs  of  glasses,  one  pair  for  distance,  the  other 
for  near  vision.  The  crystalline  lens  of  the  eye  having  been  re- 
moved, all  accommodation  is  suspended  and  the  refractive 
power  of  the  eye  is  practically  limited  to  that  of  the  cornea 
alone.  Rays  of  light  entering  such  an  eye  are  not  deflected 
sufficiently  to  meet  on  the  retina.  A  strong  convex  lens 
(from  5  D.  to  12  D.)  is,  therefore,  necessary  for  distant 
vision,  and  a  stronger  one  (from  12  D.  to  20  D.)  for  read- 
ing. There  is  usually  present,  also,  a  considerable  degree  of 
astigmia,  caused  by  the  incision  through  the  cornea  and  the 
resulting  alteration  in  its  curvature.  This  must  also  be  cor- 
rected in  the  ordinary  way  and  the  proper  cylindrical  lens 
added  to  the  spherical  lens  prescribed. 

AMBLYOPIA   AND  AMAUROSIS. 

When,  in  the  absence  of  any  visible  changes  in  the  eye, 
there  is  a  partial  loss  of  vision  which  cannot  be  improved 
by  the  use  of  glasses,  the  condition  is  known  as  Amblyopia. 


ANISOMETROPIA.  83 

When  there  is  a  complete  loss  of  vision  without  evident 
changes  in  the  structure  of  the  eye,  the  condition  is  known 
as  Amaurosis. 

Amblyopia  and  Amaurosis  may  be  the  result  of  disuse,  as 
in  the  case  of  the  squi'nting  eye,  or  it  may  be  caused  by  in- 
jury, loss  of  blood  or  cerebral  disease.  It  is  also  occasionally 
met  in  hysterical  individuals ;  in  such  cases  it  is  temporary 
and  accompanied  by  other  hysterical  symptoms. 

MALINGERING. 

For  the  purpose  of  collecting  damages  or  insurance  after 
a  trivial  injury,  or  to  gain  an  increase  of  pension,  a  patient 
may  pretend  to  be  blind.  When  complete  blindness  of  both 
eyes  is  claimed,  it  is  often  difficult  to  disprove,  except  by  ob- 
serving the  patient  without  his  knowledge.  Perhaps  the 
most  effective  method  in  such  a  case  is  by  pretending  a  blow 
over  the  eye.  If  the  patient  sees,  the  eye  will  be  suddenly 
closed  to  prevent  injury. 

When  blindness  in  one  eye  is  claimed,  several  tests  may  be 
made  to  disclose  the  true  condition.  A  ten  degree  prism, 
base  out,  may  be  placed  before  the  eye  under  suspicion.  If 
the  eye  sees,  it  will  rotate  inw^ard,  in  an  unconscious  effort 
to  fuse  the  two  images  into  one. 

If,  with  a  strong  convex  or  concave  lens  before  the  good 
eye  and  a  plane  lens  before  the  pretended  poor  e}re,  the  pa- 
tient is  able  to  read  the  Snellen  type  at  twenty  feet,  blind- 
ness is  disproved. 

It  is  needless  to  add,  perhaps,  that  the  examination  of  a 
patient  suspected  of  simulating  blindness  must  be  conducted 
without  arousing  suspicion  as  to  the  true  purpose  of  the  ex- 
aminer. The  examiner  should  act  as  if  convinced  that  blind- 
ness exists  and  that  he  is  seeking  for  its  cause. 


CHAPTER  X. 
PRESBYOPIA. 

CAUSES    AND    SYMPTOMS, 

The  hardening  of  the  lens,  previously  mentioned  in  our 
consideration  of  the  subject  of  accommodation  (p.  43),  is  a 
physiological  change,  which,  beginning  in  early  life,  shows  a 
steady  and  almost  constant  increase  as  the  age  of  the  indi- 
vidual advances.  Near  vision  becomes  more  difficult,  the 
patient  ultimately  finding  that  he  is  no  longer  able  to  see 
distinctly  at  close  range.  Gradually  he  is  compelled  to  hold 
his  book  or  newspaper  at  a  greater  distance  than  formerly 
(Fig.  53)  or  he  finds  it  necessary  when  reading  fine  print  to 
hold  it  in  a  bright  light,  the  resulting  contraction  of  the 
pupil  rendering  vision  more  distinct.  By  these  means  the 
wearing  of  glasses  for  near  work  may  be  deferred,  distant 
vision  remaining  unimpaired.  But  the  time  comes  when,  even 
by  these  means,  the  threading  of  a  needle  or  the  reading  of  a 
newspaper  is  no  longer  possible.  This  is  due  to  the  fact  that 
the  lens  of  the  eye  has  so  far  lost  its  elasticity  that  by  no 
effort  of  the  ciliary  muscle  can  the  lens  be  made  sufficiently 
convex  to  focus  on  the  retina  the  divergent  rays  of  a  nearby 
object.  This  failure  in  the  functional  activity  of  the  crystal- 
line lens  usually  becomes  manifest  between  the  ages  of  forty- 
three  and  forty-five,  although  in  some  cases  in  which  the 
eyes  are  used  but  little  for  close  work  and  the  general  health 
is  good,  it  may  not  require  correction  until  later. 

The  phenomenon  by  which  patients  of  advanced  age  are 
able  to  read  fine  print  without  glasses  is  frequently  a  source 
of  wonder  among  the  laity  and  by  them  considered  a  sign  of 
unusual  constitutional  powers.  It  signifies  merely  that  the 
individual  in  his  earlier  years  has  been  nearsighted,  but  his 
myopia  was  gradually  overcome  by  the  increasing  presby- 
opia until  the  patient  finally  had  normal  vision  for  the  first 
time.  The  eyeball  being  too  long,  the  crystalline  lens  had 


PRESBYOPIA. 


85 


55  Ye 


65  Yec 


FIG.  53. — Showing  Gradual  Recession  of  the  Near  Point  Owing  to  Harden- 
ing and  Flattening  of  the  Lens — Presbyopia. 


86  REFRACTION. 

been  able  to  focus  clearly  on  the  retina  only  very  divergent 
rays  of  close  objects.  Having  lost  its  elasticity,  the  lens  be- 
came more  flattened,  its  refractive  power  decreased  so  that 
rays  which  formerly  focussed  in  front  of  the  retina  now 
form  a  clear  image  upon  it. 

A  very  definite  relationship  exists  between  the  amount  of 
presbyopia,  or  failure  of  accommodation,  and  the  age  of  the 
patient,  as  the  following  table  will  show : 

TABLE    IV. 

SHOWING   THE    RELATIONSHIP   BETWEEN   PRESBYOPIA    AND    THE    AGE 
OF  THE  PATIENT. 

Age.  Failure  of  Accommodation. 

45 *  D. 

50 .    .  1.75  to  2.00  D. 

55 2-50  to  3.00  D. 

60 3.00  to  4.00  D. 

65 3.50  to  4.50  D. 

CORRECTION   OF    PRESBYOPIA. 

From  what  has  been  said,  it  is  evident  that  patients,  who 
until  the  age  of  forty -five  years  have  worn  no  glasses,  after 
that  time  should  be  provided  with  the  proper  correction  for 
reading.  Provided  no  astigmia  or  other  more  serious  error 
of  refraction  be  present,  such  patients  can  readily  select  a 
"reading  glass  "'from  among  an  assorted  stock  without  the 
assistance  of  a  refractionist.  From  the  very  nature  of  pres- 
byopia, it  will  be  seen  that  the  patient's  glasses  must  be 
changed  every  three  to  five  years,  their  strength  being 
steadily  increased  to  make  up  for  the  gradual  decrease  inci- 
dent to  the  flattening  of  the  crystalline  lens.  In  selecting 
glasses  for  presbyopia,  the  following  rules  will  be  helpful : 

(a)  When  no  other  error  of  refraction  exists  the  strength 
of  the  reading  glasses  will  be  determined  by  the  age  of  the 
patient  and  will  conform  approximately  to  the  table. 

(b)  When  the  patient  has  previously  worn  glasses  for 
distant  vision  on  account  of  astigmia  or  myopia  of  low  de- 
gree, the  reading  correction  prescribed  will  be  the  distance 
prescription   increased  by  the  correction  for  presbyopia  at 
the  patient's  age. 


PRESBYOPIA.  87 

(c)  When  the  patient  has  previously  worn  glasses  for 
both  distant  and  near  vision  on  account  of  myopia  of  high 
degree,  or  of  myopic  or  mixed  astigmia,  of  high  degree,  the 
reading  correction  prescribed  will  be  the  previous  reading 
lens  added  to  the  presbyopic  correction  for  the  given  age. 

Examples,  (b)  A  patient  who  had  been  wearing  —  4  D. 
S.  C  -  2.75  D.  C.  Ax.  15°  for  distance  and  -  1.50  D.  S.  C 
-  2.75  D.  C.  Ax.  15°  for  reading  would,  at  age  forty-five 
years,  require  for  reading  the  addition  of  +  1  D.  Sph.,  mak- 
ing his  prescription  read  —  .50  D.  S.  C~  2.75  D.  C.  Ax.  15°. 

(c)  A  patient  who  had  been  wearing  4  2.75  D.  C.  Ax. 
45°  C  —  1.50  D.  C.  Ax.  135°  for  distance  and  4-  4.25  D.  C. 
Ax.  45°  for  reading  would,  at  age  fifty  years,  require  for 
reading  +  2  D.  S.  C  -  4.25  D.  C.  Ax.  45°. 

In  deciding  upon  the  strength  of  glasses  for  reading,  it  is 
not  always  practicable  merely  to  add  to  the  patient's  distance 
glass  the  strength  of  lens  necessary  at  the  age  of  the  patient. 
Too  strong  a  lens  for  near  work  would  be  uncomfortable. 
Here,  as  elsewhere  in  the  practice  of  refraction,  no  hard  and 
fast  rules  can  be  followed  out,  the  patient's  comfort  being 
the  first  consideration.  A  good  rule  to  follow  is  to  take 
the  refraction  of  each  eye  separately  for  near  work,  selecting 
such  lens  or  combination  as  will  give  the  most  acute  vision. 
Then  try  the  two  eyes  together,  reducing  the  total  correction 
to  one  which  gives  good  vision,  without  causing  the  patient 
any  uncomfortable  symptoms. 


CHAPTER  XL 
CONFIRMATORY  TESTS  (OBJECTIVE). 

INTRODUCTION. 

The  tests  heretofore  described  have  been  entirely  subject- 
ive, L  e.,  dependent  in  every  case  upon  the  replies  of  the  pa- 
tient to  the  questions  of  the  examiner.  In  children  and 
illiterates,  these  tests  are  not  always  practicable,  and  object- 
ive tests  must  be  substituted.  In  fact,  as  a  matter  of 


FIG.  54. — Showing  Argand  Burner  with  Thorington  Retinoscopy  Chimney. 

routine,  it  is  advisable  to  employ  one  or  more  confirmatory 
tests  in  all  cases  of  refraction.  This  reduces  the  possibility 
of  error  to  the  minimum. 

Objective  tests  are  those  which  depend  solely  upon  the  ex- 
aminer. They  consist  essentially  of  a  series  of  maneuvers 
carried  out  in  a  dark  room  by  means  of  a  suitable  lamp  (an 
Argand  burner  is  well  suited  to  this  purpose,  Fig.  54,) 


CONFIRMATORY   TESTS. 


89 


retinoscopic  mirror  (Fig.  55)  and  objective  lens.  In  place  of 
a  retinoscope,  the  mirror  of  an  ophthalmoscope  may  be  em- 
ployed, while  an  ordinary  case  lens  of  +  16  D.  or  +  18  D. 
will  serve  the  purpose  of  an  objective  lens. 

MIRROR  TEST   (CONCAVE   MIRROR). 

Direct  Method. — The  examiner,  if  not  emmetropic,  must 
first  correct  his  own  error  of  refraction  by  means  of  glasses 
or  the  proper  lens  placed  behind  the  sight-hole  of  the  oph- 


FIG.  55. — Showing  Retinoscope. 

thalmoscope.  He  should  then  seat  himself  opposite  his  pa- 
tient, in  a  darkened  room  and  at  a  distance  of  fifteen  inches, 
directing  the  patient  to  look  toward  the  right  side  of 
the  observer's  head  when  the  right  eye  is  being  examined, 
and  to  the  left  side  if  the  other  eye.  The  fundus  is  now 
illuminated,  and  the  reflex  observed;  if  no  details  of  the 
fundus  are  visible,  the  patient  is  emmetropic.  If  any  part  of 
the  disc  or  retinal  vessels  can  be  seen,  the  patient  is  ame- 
7 


90  REFRACTION. 

tropic.  The  observer  now  moves  his  head  in  various  direc- 
tions, and  notes  the  resulting  movement  of  the  retinal  ves- 
sels; according  as  these  move  in  the  same  direction  with  the 
head  of  the  observer,  or  in  a  direction  opposite  to  the  latter, 
the  error  of  refraction  is  determined. 


Direction  of  Movement  of  Retinal  Vessels. 


Retractive  Error. 


(a)  Movement  with  in  all  directions. 

(b)  Movement  with  in  one  meridian. 

(c)  Movement  against  in   all  merid- 


Hyperopia. 

Simple  Hyperopic  Astigmia. 

Myopia. 


(d)  Movement  against  in  one  merid-    Simple  Myopic  Astigmia. 

ian  only. 

(e)  Movement  with  in  one  meridian    Mixed  Astigmia. 

and  against  in  the  other. 


If  the  image  is  blurred,  the  strongest  convex  or  the  weak- 
est concave  lens,  which  will  render  it  distinct,  is  the  measure 
of  the  error. 

Indirect  Method. — This  test  should  be  conducted  under  a 
mydriatic.  The  conditions  are  the  same  as  in  the  previous 
test,  with  the  addition  of  an  object  lens  which  is  held  as  near 
the  patient's  eye  as  possible.  Reflecting  the  light  into  the 
eye  to  be  examined  and  keeping  the  optic  disc  in  view  the 
lens  is  gradually  withdrawn  to  a  distance  of  several  inches, 
and  the  resulting  change  in  the  size  of  the  disc  noted.  The 
following  table  will  serve  to  show  the  diagnostic  significance 
of  the  change  in  the  size  and  shape  of  the  optic  disc  on  with- 
drawing the  object  lens : 


Conduct  of  the  Optic  Disc. 


(a)  If  the  disc  .remain  the  same  size 

throughout. 

(b)  If  the  disc  diminish  in  size. 

(c)  If  the  disc  diminish  in  size  in  one 

meridian  only. 


Refractive  Error. 


Emmetropia. 


Hyperopia. 
Hyperopic  Astigmia. 


CONFIRMATORY  TESTS.  91 


Conduct  of  the  Optic  Disc. 


Refractive  Error. 


(d)  If  the  disc  diminish  in  all  merid- 

ians,  but  more  so  in  one  than 
in  the  others. 

(e )  If  the  disc  increase  in  size. 

(f)  If  the  disc  increase  in  one  merid- 

ian only. 

(g)  If  the  disc  increase  in  all  merid- 

ians, but  more  so  in  one  than  in 
the  others. 


Compound  Hyperopic  Astigmia. 


Myopia. 
Myopic  Astigmia. 


Compound  Myopic  Astigmia. 


(h)  If  the  disc  increase  in  one  merid-    Mixed  Astigmia. 
ian  and  decrease  in  the  opposite. 


RETINOSCOPY  OR  SKIASCOPY  (KERATOSCOPY). 

This  test  is  known  also  as  the  shadow  test.  To  obtain 
accurate  results  by  its  use,  a  mydriatic  should  be  employed. 
The  observer  is  seated  opposite  the  patient,  at  a  distance 
of  one  meter  in  a  dark  room.  The  lamp  should  be  placed 
above  and  a  little  to  the  rear  of  the  patient,  leaving  his 
face  in  the  shadow.  The  test  frame  should  be  adjusted 
to  the  patient's  face  and  the  eye  not  under  examination 
should  be  covered  by  means  of  the  obturator.  The  pa- 
tient directs  his  gaze  at  the  observer's  forehead,  the  light  is 
reflected  into  the  eye  as  in  the  previous  tests  and  is  made  to 
pass  across  the  retina  by  slightly  rolling  the  handle  of  the 
mirror  between  the  thumb  and  forefinger,  thereby  causing 
the  mirror  to  revolve  in  the  axis  of  the  handle  or  vertical 
meridian.  Repeat  the  test,  holding  the  handle  horizontally 
and  diagonally,  noting  in  each  case: 

(a)  The  shape  and  size  of  the  shadow  surrounding  the 
light  reflex. 

(b)  The  brightness  of  the  image. 

(c)  The  relative  direction  of  the  movement  of  the  shadow 
as  compared  \vith  the  direction  of  rotation  of  the  mirror. 

(d)  The  rate  of  motion  of  the  reflex. 

Generally  speaking,  if  the  reflex  is  bright,  the  surround- 
ing shadow  narrow  or  crescentic,  and  the  rate  of  motion 
slow,  the  refractive  error  is  one  of  low  degree  and  vice  versa. 


92  REFRACTION. 

The  mirror  used  may  be  either  plane  or  concave.  When  a 
plane  mirror  is  used  the  results  are  the  reverse  of  these,  but 
the  following  are  the  results  obtained  with  a  concave  mir- 
ror: 

Emmetropia. — Theshadowmay^  show  no  apparent  move- 
ment, or  a  slight  movement  against  the  mirror  in  all  merid- 
ians. The  luminosity  is  pronounced,  and  the  edge  of  the 
shadow  is  more  or  less  crescentic. 

Hvperopia. — The  shadow  moves  against  the  mirror  in  all 
meridians.  Place  before  the  eye  convex  lenses  of  different 
strength  until  the  movement  is  reversed.  From  the  weak- 
est convex  lens  which  reverses  the  movement  deduct  -f  1  D. 
and  the  result  represents  the  measure  of  the  Hvperopia. 

Myopia  (Low  degree). — The  shadow  moves  against  the 
mirror  when  the  error  is  less  than  1  D.  Place  before  the  eye 
convex  lenses  of  different  strength  until  the  movement  is  re- 
versed. From  the  weakest  convex  lens  which  reverses  the 
movement  deduct  -f  1  D.  and  the  result  represents  the  meas- 
ure of  the  Myopia. 

Myopia  (High degree). — The  shadow  moves  with  the  mir- 
ror in  all  meridians.  Place  before  the  eye  concave  lenses 
of  different  strength  until  the  strongest  lens  is  found  with 
which  the  shadow  continues  to  move  with  the  mirror. 
From  this  likewise  deduct  ~r  1  D.  (add  —  1  D.)  and  the  result 
is  the  measure  of  the  myopia. 

Astigmia  (Hyperopic). — The  shadow  moves  against  the 
mirror  in  all  meridians.  The  weakest  convex  lens  which  re- 
verses the  movement  in  one  meridian  will  not  be  strong 
enough  to  reverse  the  movement  in  the  opposite  meridian. 
Note  the  strength  of  the  former,  then  try  stronger  lenses 
until  one  is  found  which  will  reverse  the  movement  in  the 
opposite  meridian.  By  deducting  +  1  D.  from  the  two  lenses 
which  reversed  the  movement  in  the  two  principal  meridians, 
the  refraction  is  obtained. 

Astigmia  (Myopic). —  When  of  low  degree,  the  image 
moves  against  the  mirror  in  all  meridians.  The  test  is  the 
same  as  the  preceding. 

When  the  error  is  of  high  degree  and  the  case  is  one  of 
simple  myopic  astigmia,  the  image  moves  rapidly  with  the 
mirror  in  one  meridian  and  slightly  against  in  the  other. 


CONFIRMATORY  TESTS.  93 

When  the  error  is  of  high  degree  and  the  case  is  one  of 
compound  myopic  astigmia,  the  image  moves  with  the 
mirror  in  both  meridians,  more  rapidly  in  one  than  in  the 
other.  The  strongest  concave  lens  with  which  the  shadow 
continues  to  move  with  the  mirror  in  all  meridians  is  the 
measure  of  one  of  the  principal  meridians  (after  deducting 
4-  1  D.,  i.  e.,  adding  —ID.).  Now  increasing  the  strength  of 
the  neutralizing  lens,  the  shadow  will  be  reversed  in  all  but 
one  meridian,  viz.,  the  other  principal  meridian.  The  strong- 
est concave  lens  with  which  the  shadow  continues  to  move 
with  the  mirror  is  the  measure  of  the  refraction  in  this  me- 
ridian (after  deducting  •+  1  D.,  /.  e.,  adding  —  1  D.). 

Astigmia  (Mixed). — When  the  error  in  each  of  the  princi- 
pal meridians  is  less  than  1  D.  the  image  moves  against  the 
mirror  in  all  meridians.  WThen  the  error  is  greater  than  1  D. 
the  movement  in  one  meridian  will  be  with  the  mirror,  in 
the  opposite  meridian  against  it.  The  test  is  made  like  the 
preceding  ones. 

When  astigmia  is  of  high  degree  the  retinal  reflex  has  the 
appearance  of  a  band  of  light,  so  that  the  edge  of  the 
shadow  is  straight  and  sharply  defined.  This  band  of  light 
coincides  with  one  of  the  principal  meridians  and  with  the 
axis  of  the  correcting  cylinder. 


CHAPTER  XII. 
MUSCULAR  IMBALANCE. 

OCULAR    MOVEMENTS. 

When  the  eyes  are  directed  toward  an  object  they  are 
said  to  "fix"  this  object.  When  in  this  position  the  image 
falls  upon  the  retina  of  each  eye  at  the  Macula  Lutea,  or 
point  of  most  acute  vision.  This  is  necessary  for  two  pur- 
poses, viz.:  First,  in  order  that  the  brain  perceive  but  one 
image  of  the  object  looked  at;  and,  second,  in  order  that  the 
image  be  as  clear  as  possible. 

The  act  of  fixing  is  accomplished  by  the  extrinsic  muscles 
of  the  eye,  consisting  of  four  straight  and  two  oblique  mus- 
cles, viz.,  the  Superior  Rectus,  Inferior  Rectus,  Internal 
Rectus,  External  Rectus,  Superior  Oblique  and  Inferior  Ob- 
lique. The  Recti  move  the  eyes  upward,  downward,  in- 
ward or  outward,  while  the  oblique  muscles  cause  the  eye  to 
rotate  in  the  plane  of  the  equator  of  the  eyeball.  More  than 
one  muscle  takes  part  in  each  of  these  movements,  as  is 
shown  by  the  following  table : 

TABLE  V. 

SHOWING  OCULAR  MUSCLES  CONCERNED  IN   THE  SEVERAL   EXCURSIONS  OF 

THE  EYEBALL. 


Movement. 

Direction. 

Muscles. 

Elevation. 
Depression. 

Upward. 
Downward. 

Superior  Rectus. 
Inferior  Oblique. 

Inferior  Rectus. 
Superior  Oblique. 

Adduction. 

Inward. 

Internal  Rectus. 
Superior  Rectus. 
Inferior  Rectus. 

Abduction. 

Outward. 

External  Rectus. 
Superior  Oblique. 
Inferior  Oblique. 

Sursumduction. 
Sursumduction. 


Rotation    of     the    upper  |  Superior  Oblique, 
part  of  the  eye  inward.  I  Superior  Rectus. 


Rotation    of    the     upper 
part  of  the  eye  outward. 


Inferior  Oblique. 
Inferior  Rectus. 


MUSCULAR  IMBALANCE.  95 


MUSCULAR  BALANCE  OR  ORTHOPHORIA. 

Under  normal  conditions  both  eyes  move  simultaneously 
and  to  the  same  extent.  Whether  focussed  for  near  or  dis- 
tant objects,  the  eyes  may  be  moved  up,  down,  in  or  out, 
and  in  so  doing,  both  eyes  move  at  the  same  time,  without 
altering  their  positions  as  regards  each  other.  This  is 
known  as  "associated"  movement  of  the  eyes.  When  the 
muscles  of  the  two  eyes  are  in  perfect  equilibrium  or  balance, 
the  condition  is  known  as  Orthophoria, 

The  amount  of  movement  of  which  the  normal  eye  is 
capable  is  about  45°  upward,  inward  and  outward,  and 
about  55°  downward.  This  latitude  of  motion  constitutes 
what  is  known  as  the  Field  of  Fixation. 

HETEROTROPIA   OR   STRABISMUS. 

When  the  two  eyes  are  directed  toward  the  same  object 
the  image  falls  upon  corresponding  points  of  the  two  retinae, 
so  that  the  brain  perceives  but  one,  a  condition  known  as 
Binocular  Single  Vision.  This  is,  of  course,  the  normal  state. 
When  the  two  eyes  are  not  directed  toward  the  same  object 
the  condition  is  known  as  Heterotropia,  Strabismus,  Squint 
or  Cross-eye.  The  image  in  such  a  condition  does  not  fall 
upon  corresponding  points  of  the  two  retinae,  so  that  the 
brain  perceives  the  image  from  each  retina,  and  the  patient 
has  double  vision  or  Diplopia. 

That  image  produced  on  the  retina  of  the  eye  which  fixes, 
falls  upon  the  macula  and  is,  therefore,  called  the  true  image, 
while  that  produced  on  the  retina  of  the  deviating  eye  does 
not  fall  on  the  macula  and  is,  therefore,  known  as  the  false 
image.  After  the  condition  of  Heterotropia  has*  existed  for 
some  time,  the  patient  learns  to  disregard  or  suppress  the 
false  image  and  depends  entirely  upon  the  vision  of  the  nor- 
mal eye.  The  functional  disuse  to  which  the  retina  of  the 
squinting  eye  is  subjected,  results  ultimately  in  a  loss  of  sen- 
sitiveness to  light  stimulation,  a  condition  known  as 
Amblyopia  and  one  which  cannot  be  improved  by  the  use 
of  glasses. 


96  REFRACTION. 


HETEROPHORIA. 

In  the  absence  of  an  actual  deviation  of  either  e3re,  there 
may  yet  be  present  a  muscle  imbalance  or  Heterophoria,  ow- 
ing to  the  fact  that  one  muscle  or  set  of  muscles  is  weaker 
than  another.  An  almost  constant  effort  is,  therefore,  neces- 
sary to  compel  the  weaker  muscle  to  do  as  much  work  as  its 
stronger  opponent.  This  continued  strain  leads  often  to 
pronounced  symptoms  of  asthenopia. 

Heterophoria  bears  much  the  same  relation  to  hetero- 
tropia  or  squint  as  a  latent  error  of  refraction  does  to  one 
which  is  manifest,  i.  e.,  it  is  concealed  and  overcome  by  the 
exertion  of  sufficient  muscular  and  nervous  energy,  wrhereas 
a  squint  is  manifest  and  cannot  be  overcome  by  any  mus- 
cular effort.  Heterophoria  is  a  tendency  toward  deviation, 
while  heterotropia  is  an  actual  deviation. 

FORMS  OF   HETEROPHORIA. 

The  varieties  of  Heterophoria  are : 

Esophoria,  a  tendency  of  the  eye  t'o  turn  inward. 

Exophoria,  a  tendency  of  the  eye  to  turn  outwrard. 

Hyperphoria,  a  tendency  of  the  eye  to  turn  upward. 

Hypophoria,  a  tendency  of  the  eye  to  turn  downward. 

Hyperesophoria,  a  tendency  of  the  eye  to  turn  upward 
and  inward. 

Hyperexophoria,  a  tendency  of  the  eye  to  turn  upward 
and  outward. 

Cyclophoria,  a  tendency  of  the  eye  to  turn  in  the  plane  of 
the  equator  of  the  eye. 

DETERMINATION   OF   HETEROPHORIA. 

The  pres'ence  of  latent  errors  of  refraction  was  determined 
by  making  such  errors  manifest.  This  was  done  by  over- 
coming the  activity  of  the  muscles  of  accommodation,  which 
concealed  the  underlying  conditions.  In  a  similar  way  the 
presence  of  heterophoria  is  made  manifest.  Numerous 
methods  have  been  devised  and  many  instruments  have  been 
constructed  for  the  determination  of  heterophoria,  but  all 
are  based  on  the  following  fundamental  principles: 


MUSCULAR   IMBALANCE.  97 

(a)  T$y    producing    artificial    diplopia    and  determining 
whether  the  two  images  fall  upon  corresponding  points  of 
the  two  retinae. 

(b)  By   creating  two  different  images,  one  falling  upon 
the  retina  of  one  eye,  the  second  upon  the  retina  of  the  other, 
and  determining  whether  the  two   images  fall  upon  corre- 
sponding points  of  the  two  retinas. 

For  the  purpose  of  creating  an  artificial  diplopia  strong 
prisms  are  employed.  An  object  looked  at  through  a  prism 
appears  displaced  toward  the  apex,  the  extent  of  the  displace- 
ment depending  upon  the  strength  of  the  prism.  (See  Fig. 
19,p.3O.)  This  apparent  displacement  is  due  to  the  fact  that 
light  rays  in  passing  through  a  prism  are  deflected  toward 


FIG.  56. — Showing  Apparent  Displacement  of  an  Object  Toward  the  Apex 

of  a  Prism. 

the  base.  The  eye,  perceiving  the  rays  as  they  emerge  along 
their  altered  course  from  the  prism,  sees  the  image  in  the 
direction  of  this  altered  course  (Fig.  56.)  Hence,  if  a  strong 
prism  be  placed  before  one  eye  while  the  other  eye  remains 
uncovered,  two  images  will  be  seen ;  one  image  will  be 
deflected  toward  the  apex  of  the  prism,  while  the  position 
of  the  other  remains  unchanged. 

A  double  prism,  two  prisms  placed  base  to  base,  may  be 
employed.  When  placed  before  one  eye,  two  deflected  images 
are  produced,  one  toward  the  apex  of  each  prism,  while  the 
image  of  the  uncovered  eye  will  lie  between  these  two  de- 
flected images. 

The  tests  for  Heterophoria  most  frequenth-  used  are: 
(1)  Prism  test;  (2)  Maddox  rod  test;  (3)  Cobalt  test. 


98  REFRACTION. 


PRISM   TEST. 

The  right  eye  being  uncovered,  by  placing  before  the  left  eye 
an  eight  degree  prism  with  its  base  up  or  down,  two  images 
will  be  seen,  one  above  the  other.  If  both  images  lie  in  the 
same  vertical  plane,  one  exactl}'  above  the  other,  those 
muscles  which  move  the  eyes  to  the  right  and  left  (hori- 
zontally) are  properly  balanced.  If  they  do  not  lie  one  ex- 
actly above  the  other,  the  horizontal  muscles  are  not  bal- 
anced, and  that  prism,  base  in  or  out,  which  causes  them  to 
lie  in  the  same  plane,  one  exactly  above  the  other,  is  the 
measure  of  the  heterophoria.  When  the  base  of  the  prism 
which  corrects  the  error  is  directed  inward,  the  condition  is 
one  of  Exophoria.*  If  the  base  is  directed  outward  the  con- 
dition is  Esophoria. 

Example:  If  a  four  degree  prism,  base  out,  is  found  to 
place  the  two  images  one  exactly  above  the  other,  the  con- 
dition is  one  of  Esophoria  and  its  measure  is  four  degrees. 

To  test  the  equilibrium  of  the  vertical  muscles  place  be- 
fore the  left  eyeaten  degree  prism,  base  in.  If  the  two  images 
are  on  the  same  level,  perfect  equilibrium  exists.  If  one  is 
higher  than  the  other,  heterophoria  is  present,  and  that 
prism,  base  up  or  down,  which  causes  the  two  images  to  lie 
on  the  same  level,  is  the  measure  of  the  error. 

MADDOX   TEST. 

This  test  is  best  executed  in  a  darkened  room.  Place  the 
simple  or  multiple  Maddox  rod,  Fig.  32,  before  the  left  eye, 
the  right  eye  remaining  uncovered.  Direct  the  patient  to  look 
at  a  flame  twenty  feet  distant.  The  uncovered  eye  will  per- 
ceive the  image  of  the  flame,  while  the  other  will  see  a  streak 
of  light.  When  the  Maddox  rod  is  horizontal  this  streak 
will  be  vertical  and  vice  versa. 

If  the  streak  does  not  pass  directly  through  the  flame, 
heterophoria  is  present  and  the  relative  position  of  the 
streak  to  the  flame  will  serve  to  diagnose  its  kind.  That 

*The  apex  of  the  correcting  prism,  like  the  sharp  edge  of  an  axe,  is  al- 
ways directed  toward  the  muscle  which  would  be  cut  were  the  case  one  of 
actual  deviation  or  strabismus. 


MUSCULAR   IMBALANCE. 


99 


prism  which  causes  the  streak  to  run  directly  through  the 
flame  is  the  measure  of  the  error. 

The  following  table  will  serve  to  indicate  the  diagnostic 
value  of  the  several  positions  assumed  by  streak  and  flame : 

TABLE  VI. 

SHOWING  RELATIVE   POSITION     OF     STREAK   TO   FLAME   IN   THE   DIAGNOSIS 
OF   HETEROPHORIA. 

(Maddox  Rod  over  Left  Eye. ) 


Position  of  Mad-     Direction  of  Light :       Position  of  Streak  with 
dox  Rod.  Streak.  Regard  to  Flame. 


Heterophoria. 


Horizontal. 


Vertical. 


Vertical. 


To  the  left  of  flame. 


To  the  right  of  flame. 
Horizontal.          Above  the  flame. 

Below  the  flame. 


Esophoria. 
Exophoria. 

Hypophoria 
(left  eye  turns 
down ). 

Hyperplioria 
(left  eye  turns 
up). 


COBALT  TEST. 

This  is  similar  to  the  Maddox  Rod  test,  a  blue  image  of 
the  flame  being  substituted  for  the  light  streak.  By  placing 
the  Cobalt  blue  glass  found  in  the  test  case  before  the  left 
eye,  the  right  eye  being  uncovered,  and  directing  the  patient 
to  look  at  a  flame  twenty  feet  away,  two  images  will  be  per- 
ceived, one  yellow  and  one  blue.  The  relative  positions  of  the 
two  images  will  serve  to  diagnose  the  condition.  If  the 
colored  image  is  at  the  left  of  the  true  image,  the  condition 
is  that  of  Esophoria.  If  the  colored  image  is  at  the  right, 
the  condition  is  that  of  Exophoria.  If  above,  the  left  eye 
turns  down ;  if  below,  the  left  eye  turns  up — Hyperphoria. 
This  is  a  simple  procedure  which  will  serve  to  demonstrate 
the  presence  of  heterophoria. 

COVER   TEST. 

While  one  eye  is  covered,  the  patient  should  be  directed  to 
look  at  some  object  about  twelve  inches  distant.  If,  on.  re- 


100  REFRACTION. 

moval  of  the  obturator,  the  eye  is  seen  to  move,  muscle  im- 
balance exists.  The  direction  of  this  movement  indicates  the 
weak  muscle,  for  it  was  the  weak  muscle  which  was  relaxed 
while  the  eye  was  at  rest.  When  the  eye  was  uncovered  this 
weak  muscle,  in  response  to  excessive  stimulation,  drew  the 
eye  to  its  proper  position  to  re-establish  muscular  balance. 
Hence,  if  the  eye  moves  inward  on  uncovering,  outward 
deviation  had  taken  place  while  the  eye  was  at  rest  and  the 
condition  is,  therefore,  one  of  Exophoria. 

DETERMINATION   OF   CYCLOPHORIA. 

Insufficiency  of  the  oblique  muscles  of  the  eye  is  seldom 
found  to  exist  alone,  but  usually  in  conjunction  with  an  in- 
sufficienc}'  of  one  of  the  recti  muscles.  It  is  best  determined 
by  means  of  the  double  prism  found  in  the  test  case.  This 
is  placed  before  one  eye,  the  other  eye  being  uncovered.  A 
card  on  which  a  horizontal  line  is  drawn  is  held  at  a  distance 
of  eighteen  inches.  By  the  action  of  the  double  prism  two 
lines  are  seen  parallel  to  each  other.  Between  them  is  a  third 
line,  the  image  on  the  retina  of  the  uncovered  eye.  If  the 
line  is  parallel  with  the  other  two,  perfect  balance  is  present. 
But  if  this  third  line  is  oblique  to  the  other  two,  cyclophoria 
is  indicated.  That  prism,  by  the  aid  of  which  the  three  lines 
are  made  parallel,  is  the  measure  of  the  error.  The  position 
of  the  correcting  prism  before  the  eye  will  be  oblique,  with 
the  base  in  or  out. 

CORRECTION    OF   HETEROPHORIA. 

According  to  the  degree  of  the  insufficiency,  four  methods 
are  available  for  the  relief  of  muscular  imbalance,  viz. : 

(a)  Correction  of  the  error  of  refraction. 

(b)  Prism  exercise. 

(c)  Wearing  of  correcting  prisms. 

(d)  Operation. 

Correction  of  the  Error  of  Refraction. — In  mild  cases  cor- 
rection of  the  error  of  refraction  will  be  sufficient  for  the  re- 
lief of  the  condition.  The  muscles  which  turn  the  eyes  in- 
ward are  supplied  by  the  third  nerve,  which  also  supplies  the 
ciliar\'  muscle.  This  will  explain  the  very  close  relationship 


MUSCULAR  IMBALANCE.  101 

existing  between  accommodation  and  convergence.  Any  in- 
crease or  decrease  in  the  stimulation  of  the  muscles  of  ac- 
commodation likewise  affects  the  muscles  of  convergence. 

Esophoria  is  usually  the  result  of  uncorrected  Hyperopia. 
The  constant  contraction  of  the  ciliary  muscle  in  its  effort  to 
overcome  the  hyperopia  is  accompanied  by  increased  nerve 
stimulation  of  the  internal  recti,  resulting  in  too  great  con- 
vergence. 

Exophoria  is  usually  associated  with  myopia.  The  re- 
laxation of  the  ciliar\r  muscle  being  necessar}^  to  clear  vision, 
the  muscles  of  convergence  do  not  receive  the  requisite 
amount  of  stimulation  and,  in  consequence,  relax. 

Hyperphoria  is  frequently  found  associated  with  marked 
assymetry  of  the  face  and  with  some  forms  of  anisometropia. 

Prism  Exercise. — By  means  of  persistent,  regular  and  sys- 
tematic exercise  of  the  weaker  muscle,  the  same  can  fre- 
quently be  strengthened  sufficiently  to  do  its  work  properly, 
especially  after  any  existing  refractive  error  has  been  cor- 
rected. In  conducting  prism  exercises,  the  prism  should 
always  be  inserted  in  the  position  which  actually  increases 
the  error,  i.  e.,  base  over  the  stronger  muscle.  Prism  exer- 
cise is  most  promising  of  success  in  the  treatment  of  exo- 
phoria,  less  so  in  correcting  esophoria  or  hyperphoria. 

These  exercises  are  conducted  as  follows :  Weak  prisms 
are  placed  before  the  eyes,  base  over  the  stronger  muscle,  and 
the  patient  is  directed  to  look  at  a  flame  twenty  feet  awav. 
As  soon  as  the  double  images  come  together,  the  result  of 
considerable  effort  on  the  part  of  the  patient,  the  prisms  are 
removed.  After  a  moment's  rest,  they  are  again  replaced. 
These  exercises  should  be  conducted  five  or  ten  minutes 
daily,  the  strength  of  the  prism  being  gradually  increased. 

Wearing"  of  Prisms. — When  the  kind  and  amount  of 
heterophoria  has  been  determined,  it  is  frequently  necessary 
to  assist  the  weaker  muscle  by  prescribing  a  prism  for  con- 
stant \vear.  No  hard  and  fast  rule  can  be  followed  in  deter- 
mining the  strength  of  the  prism  to  be  prescribed.  The  fol- 
lowing suggestions  will  be  useful : 

Exophoria.  —  From  one-half  to  three-quarters  of  the 
strength  of  the  correcting  prism  may  be  prescribed. 


102  REFRACTION. 

Esophoria. — When  of  low  degree  no  prism  need  be  worn. 
When  of  high  degree,  about  one-quarter  of  the  strength  of 
the  correcting  prism  may  be  prescribed. 

Hyperphoria. — The  full  correcting  prism  should  be  pre- 
scribed. 

For  the  sake  of  comfort  and  cosmetic  effect,  it  is  custom- 
ary in  prescribing  prisms  to  divide  the  prism  correction  be- 
tween the  two  lenses.  Instead  of  placing  a  four-degree 
prism  before  one  eye  with  an  ordinary  lens  before  the  other, 
two  two-degree  prisms  are  ordered.  If  the  base  of  the  pre- 
scribed prism  is  directed  inward,  both  prisms  must  be  di- 
rected inward;  if  outward,  both  prisms  are  prescribed  base 
out.  The  effect  of  a  two-degree  prism  base  out  before  each 
eye  is  the  same  as  that  of  a  four-degree  prism  base  out  before 
one  eye,  because  in  each  case  the  images  falling  on  the  two 
retinas  are  deflected  inward  an  equal  amount. 

Only  weak  prisms  can  be  worn ;  strong  prisms  have  the 
effect  of  breaking  up  the  light  which  passes  through  them, 
producing  what  is  known  as  Chromatic  Aberration.  The 
•weight  of  such  prisms  is  a  further  objection  to  them. 

When  a  patient  presents  a  refractive  error  in  addition  to 
his  heterophoria,  instead  of  prescribing  prisms  the  lens  may  be 
ordered  decentered.  (Fig.22.)  (See  "Relationship  of  Lenses," 
p.  34.)  Decentering  a  convex  lens  inward  or  a  concave 
lens  outward  produces  the  effect  of  a  prism,  base  in,  and,  of 
course,  a  convex  lens  decentered  outward,  or  a  concave  lens 
inward,  produces  the  effect  of  a  prism  base  out.  The 
strength  of  the  prismatic  effect  obtained  depends  upon  (a) 
the  curvature  of  the  surface,  /.  e.,  upon  the  strength  of  the 
lens,  and  (b)  upon  the  extent  of  the  decentration. 

Operation. — Only  in  extreme  cases,  in  which  the  suggested 
methods  have  been  tried  without  relief  to  the  patient,  should 
recourse  be  had  to  operative  procedure.  A  partial  tenotomy 
of  one  or  both  of  the  stronger  muscles  may  be  made.  This 
phase  of  the  subject,  however,  falls  beyond  the  scope  of  this 
treatise. 


CHAPTER  XIII. 
SPECTACLES  AND  EYE  GLASSES. 

INTRODUCTION. 

The  most  satisfactory  method  of  wearing  glasses  is  in 
spectacle  frames.  The  advantages  of  spectacles  over  eye 
glasses  are :  (a)  They  can  be  worn  \vhen  eye  glasses  cannot 
be  kept  in  place;  (b)  They  maintain  cylindrical  lenses  at 
their  proper  axes;  (c)  They  cannot  fall  off,  no  matter  what 
may  be  the  position  of  the  patient's  head. 

On  the  other  hand,  the  fact  that  eye  glasses,  especially  the 
rimless  variety,  are  much  less  conspicuous  and  more  becom- 
ing to  most  people,  gives  them  the  advantage  whenever  the 
cosmetic  effect  is  an  important  consideration.  Glasses  will 
be  more  cheerfully  endured  if  they  are  becoming  than  if  they 
are  not  so.  The  lighter  weight  of  eye  glasses  is  an  added  ad- 
vantage. Eye  glasses  are  of  doubtful  value  and  ordinarily 
should  not  be  prescribed  when  the  patient's  correction  calls 
for  strong  cylinders,  because  a  slight  drooping  of  the  lenses, 
by  changing  the  axes  of  the  cylinders,  alters  their  refractive 
effect. 

SPECTACLES. 

There  are  two  principal  varieties  of  spectacle  mountings, 
viz.,  those  having  metal  rims  encircling  the  lenses,  called 
"full  frames,"  and  those  without  such  rims,  known  as  "rim- 
less" or  "skeleton"  mountings. 

Full  frame  spectacles  are  more  durable  than  the  rimless, 
owing  to  the  protection  afforded  the  lenses  by  the  metal 
rims.  (Fig.  57.)  The  chief  objection  to  this  form  of  spec- 
tacle is  the  fact  that  the  rims  interfere  with  vision,  thus 
proving  a  source  of  annoyance  to  the  patient.  A  further  ob- 
jection is  the  fact  that  the  rims  render  the  frames  more  con- 
spicuous and  also  add  to  their  weight. 

The  rimless  or  skeleton  mountings  are  neater  in  appear- 
ance, less  conspicuous,  of  lighter  weight,  and  do  not  inter- 
fere with  vision.  (Fig.  58.) 


104 


REFRACTION. 


Spectacle  mountings  consist  of  a  bridge  or  nose  piece, 
two  lenses,  with  or  without  rims,  and  two  temples  or  side 
pieces  which  pass  backward  over  the  ears  and  serve  to  hold 


FIG.  57. — Showing  Full  Frame  Spectacle  Mounting. 

the  frame  in  place.  The  frames  may  be  constructed  of  gold, 
silver,  nickel  or  steel;  various  alloys  are  also  used,  the  chief 
constituent  of  which  is  tin.  Gold,  by  virtue  of  its  superior 
wearing  qualities,  has  proved  most  satisfactory.  Silver 


FIG.  58. — Showing  Rimless  or  Skeleton  Spectacle  Mounting. 

frames  are  soft,  easily  bent  out  of  their  proper  shape,  and 
too  readily  tarnish,  while  steel  and  nickel  plated  frames  are 
liable  to  rust. 


SPECTACLES  AND   EYE   GLASSES.  105 

REQUISITES   OF   PROPERLY   FITTING   GLASSES. 

For  glasses  to  occupy  their  proper  position,  and  to  be 
worn  with  comfort,  the  following  conditions  must  be  met : 

1.  The  optical  center  of  each  lens  must  be  directly  in 
front  of  the  pupil  of  each  eye. 

2.  The  lenses  must  be  so  far  removed  from  the  eyes  that 
the  patient's  lashes  do  not  come  in  contact  with  them. 

3.  Both  lenses  must  lie  in  the  same  plane  and  this  plane 
must  be  at  right  angles  to  the  direction  of  vision. 

4.  The  long  diameter  of  the  lenses  must  be  horizontal. 

5.  The  lenses  must  be  large  enough  to  afford  an  ample 
field  of  vision. 

6.  The  bridge  or  nose-piece  of  spectacles  must  be  of  the 
proper  height  and  width  to  lie  flush  with  the  nose,  and  the 
temples  must  be  long  enough  to  pass  loosely  over  the  ears, 
and  far  enough  apart  to  avoid  any  pressure  on  the  sides  of 
the  face. 

7.  The  guards  of  eye  glasses  must  lie  flush  with  the  sides 
of  the  nose  and  must  neither  irritate  nor  exert  undue  press- 
ure upon  the  tissues. 

MEASUREMENTS    AND  DATA   FOR  ORDERING  SPECTACLES. 

In  ordering  spectacle  frames  it  is  necessary  that  the 
manufacturer  be  furnished  with  certain  data  and  measure- 
ments. These  are : 

(a)  The  prescription  for  each  lens. 

(b)  The  kind  of  frame,  whether  full  or  rimless. 

(c)  The  material  of  construction,  whether  gold,  silver, 
nickel,  steel  or  alloy. 

(d)  The  interpupillary  distance. 

(e)  The  size   of  the  lenses  desired  and  their  angle,  i.  e., 
whether  vertical  or  with  their  upper  edges  tilted  forward. 

(f)  The  dimensions,  the  height  and  breadth,  of  the  bridge 
or  nose  piece. 

(g)  The  form  of  the  temples,  whether  straight  or  riding 
bows.    When  the  latter  are  ordered  the  kind  must  be  speci- 
fied, whether  plain  or  cable. 

(h)   The  length  of  the  temples. 
8 


106  REFRACTION. 

Interpupillary  Distance.  —  This  is  the  distance  between 
the  two  pupils.  It  is  best  measured  by  holding  a  small 
ivory  rule  before  the  patient's  eyes,  Fig.  59,  and  noting  the 


FIG.  59.— Showing  Method  of  Measuring  Interpupillary  Distance. 


distance  between  the  outer  edge  of  one  pupil  and  the  inner 
edge  of  the  other,  or  from  the  center  of  one  to  the  center  of 
the  other.  When  the  iris  is  very  dark,  or  the  illumination 


SPECTACLES   AND   EVE   GLASSES. 


107 


poor,  it  is  sometimes  easier  to  measure  the  distance  from  the 
outer  corneo-scleral  junction  of  one  eye  to  the  inner  corneo- 
scleral  junction  of  the  other.  In  the  manufacture  of  the 
frames  the  distance  between  the  optical  centers  of  the  lenses 
must  correspond  with  this  measurement. 

Size  of  Lenses.  The  majority  of  manufacturers  now  con- 
form to  one  standard  S3rstem  of  designating  the  size  of 
lenses.  The  latter  are  numbered,  beginning  with  the  small- 
est: 4,  3,  2,  1,  0,  00,  000,  0000  and  Jumbo.  (See  Fig.  60.) 
The  sizes  most  commonly  used  are:  For  male  adults,  00 
and  000 ;  for  female  adults,  1,  0  and  00  and  for  children  2,  1 
and  0. 


000  EYE 
41x33 

FIG.  60. — Showing  Standard  L,ens  Sizes. 

Angle  of  Lenses. — When  the  glasses  are  to  be  worn  for 
distant  vision  only,  the  lenses  should  lie  in  a  vertical  plane, 
i.  e.,  at  right  angles  to  the  direction  of  vision. 

When  the  glasses  are  prescribed  for  reading,  the  frames 
should  be  tilted  to  lie  in  a  plane  more  nearly  parallel  with 
the  patient's  book,  see  Fig.  36.  The  lenses  should  also  be 
from  a  sixteenth  to  an  eighth  of  an  inch  lower  than  for  dis- 
tance, because  the  eyes  are  directed  downward.  This  lower- 
ing of  the  lenses  is  effected  by  increasing  the  height  of  the 
bridge. 

Bridge  or  Nose  Piece. — This  is  best  measured  by  trying 
on  a  sample  frame  and  making  the  needed  corrections.  The 
width  of  the  bridge  is  the  distance  between  its  lower  ex- 
tremities, while  the  height  is  the  vertical  distance  between 


108  REFRACTION. 

the  crest  of  the  bridge  and  the  pupillary  line  (an  imaginary 
line  passing  horizontally  through  the  center  of  each  pupil). 

The  distance  of  the  lenses  from  the  eyes  is  regulated  by 
the  crest  of  the  bridge.  To  place  the  lenses  farther  from  the 
ej'es,  the  crest  of  the  bridge  is  ordered  "inset,''  /.  e.,  it  is 
made  to  lie  behind  the  plane  of  the  lenses.  To  place  the 
lenses  nearer  the  eyes  the  crest  of  the  bridge  is  ordered  "  out- 
set," i.  e.,  it  is  made  to  lie  in  front  of  the  plane  of  the  lenses. 
Prominent  eyes  and  small,  flat  noses  require  an  inset,  while 
deep-set  eyes  and  prominent  noses  require  an  outset. 

Temples. — These  are  the  side  pieces  which  hold  the  frame 
in  place.  For  reading,  straight  temples  may  be  ordered.  In 
the  majority  of  cases,  however,  riding  bows  will  be  prefer- 
able. These  pass  backward  over  and  around  the  ear.  Plain 
riding  temples  are  made  of  stiff,  tempered  wire,  while  cable 


FIG.  61. — Showing  Rimless  or  Skeleton  Bye  Glass  Mounting. 

temples  are  spirally  wound  with  a  narrow,  thin,  metal  strip, 
making  them  more  flexible  and  less  liable  to  exert  pressure 
on  the  ear. 

Temples  are  usually  made  in  three  lengths,  viz.,  short, 
medium  and  long,  these  being  respectively  five  and  one-half, 
six,  six  and  one-half  inches  in  length.  Short  temples  are 
required  for  children  and  medium  temples  for  the  average 
adult.  Only  when  the  head  is  unusually  large  need  long 
temples  be  supplied. 

EYE  GLASSES. 

There  are  two  principal  varieties  of  eye  glasses,  viz.,  the 
full  and  the  rimless  frames,  corresponding  to  the  two  varie- 
ties of  spectacle  frames  already  described.  The  rimless 


SPECTACLES  AND   EYE   GLASSES.  109 

mounting,  on  account  of  its  lightness,  simplicity  and  beaut}-, 
is  usually  preferred.     (Fig.  61.) 

Eye  glasses  consist  of  two  lenses,  a  spring,  two  studs  and 
two  nose  guards.  The  frames  are  constructed  of  the  same 
material  as  that  used  in  the  manufacture  of  spectacles. 

MEASUREMENTS   AND   DATA    FOR   ORDERING   EYE   GLASSES. 

In  ordering  eye  glasses  the  following  data  and  measure- 
ments must  be  furnished  the  manufacturer : 

(a)  The  prescription  for  each  lens. 

(b)  The  kind  of  frame,  full  or  rimless. 

(c)  The  material  of  construction. 

(d)  The  interpupillary  distance. 

(e)  The  size  of  the  lens  desired. 


FIG.  62. — Showing  Full  Frame  Eye  Glass  Mounting. 

(f )  The    kind   of  guards,  and  their  angle  with  the  plane 
of  the  lenses. 

(g)  The  distance  between  the  upper  ends  of  the  guards, 
(h)   The  distance  between  the  lower  ends  of  the  guards. 

( i )    The  shape  and  length  of  the  spring. 

( j  )  Whether  or  not  a  handle  or  hole  for  cord  or  chain  is 
desired. 

Springs.  —  The  spring  connects  the  two  halves  of  the 
frame.  (Fig.  62.)  It  may  be  either  vertical  or  horizontal. 
Vertical  springs  are  made  in  two  shapes,  the  one  circular, 
called  a  "hoop,"  the  other  elliptical,  called  the  "oblong" 
spring,  the  choice  between  them  lying  with  the  patient. 

Vertical  or  regular  springs  are  either  short,  medium  or 
long.  Medium  springs  are  suited  to  the  majority  of  cases. 
Only  in  exceptional  cases  are  short  or  long  springs  required. 


110  REFRACTION. 

Studs.— These  are  the  small  metal  pieces  which  serve  to 
hold  the  spring  and  guards  together  and  to  join  them  to  the 
lenses.  They  are  made  of  different  lengths  for  the  purpose  of 
regulating  the  distance  between  the  lenses  to  make  this  coin- 
cide with  the  interpupillary  distance.  Offset  studs  are  de- 
signed to  place  the  lenses  in  a  plane  in  front  of  the  spring. 
This  is  necessary  in  patients  having  prominent  eyes  and  long 
lashes  in  which  the  guards  do  not  remove  the  lenses  far 
enough  from  the  eyes. 

Guards. — These  are  the  two  pieces  which  rest  against  the 
sides  of  the  nose.  They  are  measured  by  placing  a  sample 
frame  on  the  patient's  nose  and  ascertaining  the  distance  be- 
tween the  two  upper  ends  and  that  between  the  two  lower 
ends. 

There  are  three  principal  varieties  of  guards,  viz. : 

(a)  Shell-faced  guards,  in  which  the  bearing  surface,  or 
that  which  lies  against  the  nose,  is  covered  with   tortoise 
shell  or  celluloid. 

(b)  Cork-faced  guards,  in  which  this  surface  is  covered 
with  cork. 

(c)  All-metal  guards.    The  latter  are  most  modern,  and 
in  the  majority  of  cases  the  most  satisfactory,  because:   1. 
They  do  not  become  foul  with  the  accumulation  of  perspira- 
tion and  dirt,  as  do  the  others;   2.  The}'  are  made  with  a 
wide  bearing  surface  to  lessen  the  irritation  on  the  nose,  and 
3.  They  are  much  neater  in  design. 

The  position  of  the  lenses  is  largely  regulated  by  the 
guards.  To  set  the  lenses  out  sufficiently  to  prevent  the 
lashes  from  coming  in  contact  with  them,  "off-set"  guards 
are  used,  the  arms  of  which  place  the  nose-pieces  or  shanks 
back  of  the  plane  of  the  lenses.  The  amount  of  off-setting 
depends  upon  the  length  of  the  arms. 

To  hold  the  lenses  in  a  perpendicular  position  for  distant 
vision,  or  to  tilt  them  for  reading,  the  arms  are  attached  to 
the  shanks  at  varying  angles.  Sample  glasses  fitted  with 
guards  at  different  angles  should  be  tried  until  one  is  found 
which  meets  the  requirements  of  the  case. 

When  it  is  desired  to  lower  the  lenses,  guards  of  the 
proper  angle  and  off-set,  with  arms  attached  to  the  lower 


SPECTACLES   AND    EYK    GLASSES.  Ill 

end  ot  the  shank,  should  be  ordered.  The  lenses  of  eye 
glasses  may  be  still  further  lowered  by  "dropping"  them, 
that  is,  by  attaching  them  to  the  frame  above  the  pupillary 
line;  or  in  the  case  of  rimless  eye  glasses,  by  boring  the  holes 
for  studs  above  the  horizontal  axis. 

OTHER   FORMS   OF   GLASSES. 

In  addition  to  the  above  described  regular  forms  of  spec- 
tacles and  eye  glasses,  there  are  a  number  of  modifications 
designed  to  meet  the  special  requirements  of  certain  patients. 
The  most  important  among  these  are  the  Half  Oval  Eye 
Frames,  Reversible  Spectacles,  Hook  or  Grab  Fronts  and 
Clerical  Glasses. 

Half  Oval  Eve  Frames. — This  form,  Fig.  63,  is  convenient 


FIG.  63. — Showing  Half-Oval  Eye  Glass  Frame. 

when  no  distant  correction  is  used  and  when  the  patient's 
occupation  requires  correction  for  near  work  alternately 
with  distant  vision.  These  frames  are  used  by  teachers, 
preachers  and  those  engaged  in  certain  kinds  of  office  work. 

Reversible  Spectacles. — When  a  patient  has  sight  in  only 
one  eye  and  this  eye  requires  correction  for  both  distant  and 
near  vision  these  frames  are  useful.  It  is  fitted  with  a  double 
bridge.  The  lens  for  distant  vision  is  placed'  on  one  side, 
that  for  reading  on  the  other.  To  change  from  the  dis- 
tant lens  to  that  for  near  work  the  patient  has  merely  to  re- 
verse the  frame,  i.  e.,  to  put  them  on  upside  down.  This 
form  of  spectacle  is  especially  useful  to  old  people  having  one 
aphakic  eye,  the  other  being  cataractous. 

Hook    or   Grab    Fronts.  —  These    frames,  as    the    name 


112  REFRACTION. 

indicates,  may  be  hooked  over  a  spectacle  frame  for  read- 
ing. They  are  fitted  with  lenses  of  such  strength  that, 
when  added  to  the  distance  lenses,  the  combined  effect  is  the 
patient's  reading  correction. 

Clerical  or  Adjustable  Eye  Glasses. — These  are  useful  for 
lecturers,  ministers  and  others  who  are  compelled  to  remove 
their  glasses  frequently.  The  guards  lie  in  the  plane  of  the 
lenses  and  readily  adjust  themselves  to  the  nose. 


CHAPTER  XIV. 
SPECIAL  FORMS  OF  LENSES. 

BIFOCAL  LENSES. 

When  the  patient  requires  correction  of  both  distant  and 
near  vision,  the  necessity  of  two  pairs  of  glasses  may  be  ob- 
viated by  prescribing  bifocal  lenses.  The  original  bifocal,  in- 
vented by  Benjamin  Franklin,  consisted  of  two  half-oval 
lenses,  the  upper  half  being  the  patient's  correction  for  dis- 
tant vision,  the  lower  half  the  correction  for  reading.  This 
form  is  now  known  as  the  "split"  bifocal.  (No.  3  of  Fig.  65.) 

A  great  many  other  forms  have  been  developed,  all  based 
on  the  original  idea  of  affording,  in  one  frame,  both  distant 


FIG.  64. — Showing  Grooved  Bifocal  Lens. 

and  reading  correction.  In  some  of  these  the  reading  correc- 
tion is  ground  on  the  lower  part  of  the  distance  lens,  in 
others  a  small  reading  lens  fits  into  a  notch  cut  out  of  the 
distance  lens  (Fig.  64),  while  in  a  third  form  a  small  "wafer" 
lens  is  cemented  on  the  back  or  front  of  the  distance  lens 
(No.  1  and  2  of  Fig.  65).  This  wafer  is  of  such  a  strength 
that,  when  added  to  the  distance  glass,  the  total  refractivity 
is  the  correction  for  reading.  This  form,  known  as  the 
cement  bifocal,  is  the  most  satisfactory  and  is  now  almost 


114 


REFRACTION. 


universally  prescribed.  The  size  and  shape  of  the  wafer 
must  be  suited  to  the  taste  and  requirements  of  the  patient. 

Although  bifocal  lenses  are  a  great  convenience,  they  pre- 
sent several  objectionable  features.  The  most  serious  of 
these  lies  in  the  fact  that  the  patient,  when  looking  at  the 
floor,  when  ascending  and  descending  stairs,  and  getting  in 
and  out  of  vehicles,  is  confused  by  the  blurring  effect  of  the 
reading  wafer.  At  such  times  he  is,  therefore,  compelled  to 
look  under  his  glasses,  or  over  the  upper  edge  of  the  wafer. 

The  edge  of  the  wafer,  in  a  properly  adjusted  spectacle 
frame,  is  below  the  pupil,  in  order  that  the  patient's  distant 
vision  be  unimpaired.  The  wafer  may  be  decentered  some- 
what to  increase  the  field  of  distant  vision.  But  even  with  the 
most  perfect  adjustment,  the  bifocal  lens  is  frequently  a  great 


FIG.  65. — Showing  Different  Forms  of  Bifocal  L,enses. 

• 

annoyance  to  the  patient.  The  annoyance  of  the  wafer  is 
partially  overcome  by  constructing  the  distance  lens  of  two 
thin  lenses,  each  having  a  groove  in  its  lower  part  and  then 
cementing  these  two  lenses  together,  their  plane  surfaces  in 
apposition.  The  reading  wafer  is  then  inserted  into  the 
groove  between  the  two  lenses.  This  form  is  known  as  the 
"invisible"  bifocal.  In  the  heavy  glasses  required  in  cases  of 
aphakia  this  is  the  only  practicable  arrangement.  Its  chief 
objection  lies  in  the  expense  of  manufacture. 

PERISCOPIC   LENS. 

A  periscopic  lens  is  one  which  is  convex  on  one  side  and 
concave  on  the  other.  The  concave  surface  has  a  spherical 
curvature  of  1.25  D.  This  curvature,  conforming  closely  to 
the  arc  of  rotation  of  the  eye,  brings  the  border  of  the  lens 


SPECIAL  FORMS  OF  LENSES.  115 

nearer  the  eye  and  does  away  with  the  large  gap  between 
the  edge  of  the  lens  and  the  face.  In  this  way  the  field  of 
vision  is  enlarged,  and  the  refracting  surfaces  of  the  lens  are 
maintained  at  right  angles  to  the  line  of  vision,  irrespective 
of  the  movements  of  the  eye.  For  these  reasons,  a  periscopic 
lens  is  to  be  preferred  to  the  ordinary  flat  lens,  only  the  cen- 
tral part  of  which  is  optically  correct.  Objects  seen  through 
the  outer  portions  of  the  common  lens  appear  more  or  less 
distorted,  a  phenomenon  known  as  Spherical  Aberration ; 
this  is  more  apparent  in  the  stronger  convex  lenses.  There 
is  in  a  flat  lens  also  a  prismatic  effect  which  increases  the 
more  the  eye  is  turned  from  the  centre  of  the  lens.  This  is 
due  to  the  fact  that  the  line  of  vision  is  not  at  right  angles 
to  the  refracting  surface.  These  objections  are  removed  by 
the  use  of  the  periscopic  lens. 

Meniscus  Lens. — When  a  periscopic  lens  has  a  deeper  con- 
cave surface  than  1.25  D.  and  a  spherical  refractive  effect,  it 
is  called  a  Meniscus.  A  converging  meniscus  is  one  in  which 
the  convex  surface  has  a  greater  curvature  than  the  concave, 
(No.  6  of  Fig.  17)  having,  therefore,  the  refractive  effect 
of  a  convex  lens.  A  diverging  meniscus  is  one  in  which  the 
concave  surface  has  a  greater  curvature  than  the  convex, 
(No.  7  of  Fig.  17)  having  therefore,  the  refractive  effect  of  a 
concave  lens. 

TORIC    LENS. 

A  toric  surface  is  one  which  is  shaped  like  the  bowl  of  a 
spoon,  i.  e.,  with  one  of  the  principal  meridians  more  convex 
than  the  other.  In  the  case  of  a  spoon,  the  meridian  lying 
crosswise  of  the  bowl  is  more  convex  than  the  meridian  ly 
ing  lengthwise  of  the  bowl.  The  convex  side  of  the  bowl 
represents,  crudely,  a  convex  toric  surface ;  the  concave  side, 
a  concave  toric  surface. 

The  term  "Toric"  is  derived  from  the  latin  "tor,"  signi- 
fying the  base  of  an  architectural  column.  Such  a  base  has 
two  curvatures,  the  horizontal  one  being  the  arc  of  a  larger 
circle  and  hence  less  convex  than  the  vertical  one.  Making 
a  vertical  section  of  such  a  tor,  /.  e.,  cutting  off  a  segment, 
we  have,  roughly  speaking,  a  plano-convex  toric  lens.  (Fig. 


116 


REFRACTION. 


66.)     By  grinding  into  a  flat  block  a  surface  conforming  to 
the  tor,  we  have  a  plano-concave  toric  lens.     (Fig.  66.) 

A  toric  lens  (Fig.  67)  is  usually  ground  with  the  toric  sur- 


FIG.  66. — Showing  a  Tor  with  Sections  to  Represent  Piano- 
Convex  and  Piano-Concave  Toric  Lenses. 

face  on  the  anterior  side,  the  posterior  surface  being  a  con- 
cave sphere  of  either  3  D.,  4.50  D.,  6  D.  or  9  D.  strength.  It 
is  used  exclusively  for  the  correction  of  simple,  compound,  or 
mixed  astigmia. 


FIG.  67. — Showing  a  Toric  Lens. 

As  has  been  stated,  one  of  the  principal  meridians  of 
a  toric  surface  has  a  stronger  refractive  power  than  has  the 
other.  The  weaker  is  called  the  "  base  curve  "  and  is  always 
in  the  same  meridian  as  the  axis  of  the  cylinder  prescribed. 


SPECIAL  FORMS  OF  LENSES.  117 

When  the  base  curve  has  the  same  refractive  power  as 
that  of  the  concave  spherical  surface,  there  will  be  no  refrac- 
tion in  this  meridian.  The  lens  will,  therefore,  have  the  ef- 
fect of  an  ordinar}'  cylinder,  the  axis  of  which  coincides  with 
the  meridian  of  the  base  curve. 

Example.  •+  1  D.  C.  Ax.  90°  in  toric  form  may  be  ground 
as  follows : 

Anterior  surface  —  Base  curve :  plus  6  D.  in  the  meridian 
of  90°  ;  4  7  D.  in  the  meridian  of  180°. 

Posterior  surface  —  6  D.  Sph. 

When  the  base  curve  has  a  different  refractive  power  than 
that  of  the  concave  spherical  surface,  the  lens  is  a  sphero- 
cylinder,  and  the  difference  in  refractivit3r  between  the  base 
curve  and  the  concave  surface  determines  the  strength  of  the 
spherical  part  of  the  lens. 

Example.  +  1  D.  S.  C  +  -50  D.  C.  Ax.  90°  in  toric  form 
may  be  ground  as  follows : 

Anterior  surface  —  Base  curve:  -r  7  D.  in  the  meridian  of 
90° ;  +  7.50  D.  in  the  meridian  of  180°. 

Posterior  surface  —  6  D.  Sph. 

One  great  advantage  of  toric  as  well  as  periscopic  lenses, 
over  the  ordinary  flat  lens,  is  their  lessened  weight.  This 
feature  is  especially  valuable  in  the  prescribing  of  cataract 
lenses  or  those  having  a  strong  spherical  element,  with  or 
without  a  cylinder.  This  is  owing  to  the  fact  that  the  curves 
are  equally  divided  between  the  two  sides  of  the  lens.  The 
spherical  aberration  of  such  a  lens  is  less  than  in  one  having 
the  entire  curve  on  one  surface. 

In  the  prescribing  of  toric  lenses,  it  is  well  to  use  an  inset 
stud,  thus  taking  advantage  of  the  hollow  of  the  lens  to 
bring  the  latter  as  near  the  eye  as  possible. 


APPENDIX. 
MECHANICAL  AIDS  TO  REFRACTION. 

A  number  of  instruments  mentioned  in  the  text,  and 
others  considered  of  value  in  confirming  the  results  of  the 
test  case,  are  here  described. 


FIG.  68. — Showing  Rear  View  of  Ophthalmoscope. 


APPENDIX.  119 


OPHTHALMOSCOPE. 

Since  the  invention  of  the  ophthalmoscope  by  von  Helm- 
holz  some  fifty  years  ago,  the  instrument  has  been  greatly 
improved  and  is  made  in  many  different  forms.  The  Loring 
pattern  (Fig.  68)  typifies  the  modern  instrument.  It  con- 
tains two  independent  rotatable  discs,  the  lower  disc  carry- 
ing fifteen  lenses,  and  the  superposed  disc  four  supplementary 
lenses.  By  combining  the  lens  powers  in  both  discs,  all  of 
the  positive  and  negative  equivalents  from  .50  D.  to  24  D. 
are  quickly  obtained  and  automatically  recorded,  the  posi- 
tive and  negative  quantities  being  expressed  in  white  and 
red  figures  respectively.  The  lenses  in  the  ophthalmoscope 
are  used  principally  for  the  neutralization  of  errors  in  the  re- 
fraction of  the  observed  eye,  and  hence  the  acquirement  of  a 
clear  view  of  the  intra-ocular  tissues.  The  use  of  the  oph- 
thalmoscope for  determining  errors  of  refraction  has  been 
fully  outlined.  (See  Objective  Tests.) 


LUMINOUS  OPHTHALMOSCOPE. 

In  this  instrument  (Fig.  69)  the  light,  instead  of  being  re- 
flected into  the  eye  from  a  wall  lamp,  as  with  the  ordinary 
ophthalmoscope,  is  supplied  by  a  small  electric  lamp  encased 
within  the  handle.  It  passes  through  a  strong  condensing 
lens  and  is  then  reflected  into  the  observed  eye.  The  ex- 
aminer looks  above  the  edge  of  the  mirror,  thus  avoiding 
the  usual  corneal  reflex  which  frequently  embarrasses  a  clear 
view  of  the  fundus. 

This  form  of  instrument  has  several  advantages  over  the 
ordinary  form,  viz.: 

1.  It  can  be  used  in  either  daylight  or  darkness. 

2.  It  affords  a  larger  field  of  view. 

3.  It  can  be  brought  very  close  to  the  eye. 

4.  It  can  be  used  at  the  bedside. 


120 


REFRACTION. 


The  chief  objection  to  this  form  of  instrument  as  com- 
pared with  the  ordinary  type  is  the  time  and  trouble  re- 
quired to  keep  it  in  perfect  working  order. 


FIG.  69. — Showing  Luminous  Ophthalmoscope. 
LUMINOUS   RETINOSCOPE. 

In  the  Luminous  Retinoscope,  as  in  the  case  of  the  Lumin- 
ous Ophthalmoscope,  the  source  of  light  is  combined  with 
the  instrument  (Fig.  70).  An  electric  lamp  is  encased  in  a 
tube  facing  a  diagonally  placed  mirror  by  means  of  which 


APPENDIX.  121 

the  light  is  reflected  into  the  eye.  The  use  of  the  retinoscope 
for  diagnosing  refractive  errors  is  described  under  Retin- 
oscopy.  (P.  91.) 


FIG.  70. — Showing  Luminous  Retinoscope. 

The  advantages  claimed  for    this  instrument  over  the 
ordinary  form  are : 

1.  It  can  be  used  in  daylight. 

2.  It  has  stronger  illumination. 

3.  It  is  portable. 
9 


122 


REFRACTION. 


THE    OPHTHALMOMETROSCOPE. 

This  instrument  (Fig.  71)  consists  of  a  Luminous  Oph- 
thalmoscope with  the  addition  of  an  illuminated  test  object 
of  multiple  radiating  lines,  so  arranged  as  to  be  projected 


FIG.  71. — Showing  Ophthalmometroscope. 

and  focussed  upon  the  retina.  The  rays  from  a  small  electric 
lamp  are  gathered  by  a  strong,  adjustable  condensing  lens 
directly  over  it,  and  projected  upon  the  translucent  test  ob- 
ject. The  light  from  the  illuminated  test  object  is  directed 


APPENDIX.  123 

by  the  obliquely  fixed  reflector  into  the  eye,  where,  by  looking 
through  the  peep  aperture  above  the  mirror,  a  perfect  repro- 
duction of  the  test  object  may  be  seen  upon  the  retina. 

In  applying  the  instrument  for  the  measurement  of  errors 
of  refraction  the  operator  moves  the  test  object  up  or  down 
in  the  tube,  as  required  to  focus  it  sharply  on  the  retina,  em- 
ploying at  the  same  time  such  lens  power  at  the  peep  aper- 
ture as  may  be  found  necessary  to  give  a  clear  view  of  the 
fundus. 

In  Emmetropic  eyes  all  the  radiating  lines  of  the  test  ob- 
ject will  appear  uniform  and  most  distinct  with  the  index  at 
zero. 

In  Simple  ftyperopia  all  the  radiating  lines  will  appear 
uniform  and  most  distinct  with  the  index  upon  the  scale  of 
white  figures,  which  is  below  zero. 

In  Simple  Myopia  all  of  the  radiating  lines  will  appear 
uniform  and  most  distinct  when  the  index  is  upon  the  scale 
of  red  figures,  which  is  above  zero. 

In  Simple  Astigmia  the  radiating  lines  in  one  meridian 
will  be  in  focus  at  zero,  while  those  in  the  opposite  meridian 
will  appear  blurred,  and  if,  to  focus  the  blurred  lines,  the  test 
object  is  moved  down,  the  error  is  Simple  Hj-peropic  As- 
tigmia, while  if  moved  up,  Simple  Myopic  Astigmia. 

In  Compound  ttyperopic  Astigmia  the  Hyperopia  is 
equal  to  the  smallest  number  among  the  white  figures,  indi- 
cating the  focus  of  the  lines  in  one  meridian,  and  the  astig- 
mia  to  the  difference  between  this  number  and  that  greater 
one  on  the  same  scale,  at  which  the  lines  in  the  opposite  me- 
ridian are  focussed. 

In  Compound  Myopic  Astigmia  the  same  conditions  pre- 
vail, excepting  that  the  foci  of  both  meridians  are  recorded 
upon  the  scale  of  red  figures. 

In  Mixed  Astigmia  the  focus  of  one  meridian  is  recorded 
by  the  white  figures,  and  that  of  the  opposite  meridian 
upon  the  red  ones. 

To  obtain  the  best  results  with  this  instrument  the  pupil 
should  be  dilated. 


124 


REFRACTION. 


FIG.  72. — Showing  Ametropometer. 


APPENDIX. 


125 


AMETROPOMETER. 

This  instrument  (Fig.  72)  consists  essentially  of  a  metal 
tube  and  a  disc  fitted  with  a  double  prism  which  can  be  re- 
volved to  any  desired  axis.  A  metal  target,  bearing  a  white 
ring  on  a  black  background,  is  hung  at  a  prescribed  dis- 
tance. The  patient  is  instructed  to  place  his  eye  close  to  the 
eye-piece,  his  head  being  steadied  by  means  of  a  chin  rest. 
Directing  his  gaze  toward  the  target  he  will  see  two  white 
rings,  their  relative  positions  in  the  different  meridians  being 
determined  by  revolving  the  metal  disc.  The  position  of  the 
two  rings  in  the  several  meridians  serves  to  diagnose  the  re- 
fractive condition  of  the  eye  examined. 

REFKACTOMETER    (OEZENG's). 

This  instrument  (Fig.  73)  is  designed  for  the  purpose  of 
measuring  refractive  errors  and   is  adapted    especially  to 


FIG.  73. — Showing  Refractonieter. 

cases  in  which  a  cycloplegic  is  contraindicated.     It  consists 
of  two  metal  tubes,  one  sliding  within  the  other,  and  regu- 


126  REFRACTION. 

lated  by  a  rack  and  pinion  adjustment.  The  outer  tube  con- 
tains a  concave  lens  of  20  D.  strength,  the  inner  one  a  con- 
vex, achromatic  lens.  These  lenses,  at  different  distances 
from  each  other,  have  a  combined  refractive  effect  varying 
from  +  .12  D.  to  +  18  D.  and  from  —  .12  D.  to  —  9  D.  The 
convex  spherical  effects  are  recorded  on  a  revolving  dial  at 
the  side,  the  concave  effects  on  the  top  of  the  inner  tube.  At 
the  front  of  the  tube  is  a  revolving  head,  composed  of  two 
revolving  discs,  containing  a  stenopseic  slit  and  a  number  of 
concave  cylinders. 

The  best  method  employed  in  conducting  a  test  is  that 
known  as  "fogging,"  /.  e.,  overcorrecting  a  hyperopic  or  un- 
dercorrecting  a  myopic  eye.  This  causes  the  type  to  blur 
and  the  accommodation  to  relax,  so  that  the  latent  errors 
are  made  manifest.  By  adjusting  the  instrument  until  the 
vision  is  clear  the  amount  of  refractive  error  is  readily 
noted.  By  fogging  and  again  clearing  the  vision  while  the 
patient's  gaze  is  directed  toward  the  astigmic  dial,  the  pres- 
ence of  latent  as  well  as  manifest  astigmia  can  be  deter- 
mined. When  astigmia  is  present  it  can  be  measured  and 
the  axis  verified  by  the  use  of  the  cylindric  lenses  contained 
in  the  revolving  discs. 

PLACIDO'S   DISC   OR    KERATOMETER. 

Placido's  disc  consists  of  a  target  attached  to  a  handle 
and  pierced  through  the  centre  by  a  small  sight-hole  (Fig.  74). 
On  the  reverse  side  is  an  attachment  for  receiving  lenses.  To 
use  the  disc  the  patient  is  placed  with  his  back  to  a  strong 
light.  The  examiner  holds  the  disc  with  the  sight-hole  di- 
rectly in  front  of  his  eye,  and,  with  the  target  brightly 
illuminated,  approaches  the  eye  to  be  examined  until  the 
outer  edge  of  the  image  on  the  cornea  coincides  with  the 
corneo-scleral  junction  of  the  examined  eye.  By  placing  a 
convex  spherical  lens  of  3  D.  or  4  D.  behind  the  sight-hole, 
the  image  will  be  magnified,  when,  if  astigmia  be  present, 
the  black  and  white  circles  will  appear  oval.  This  test  is  of 
use  in  determining  corneal  astigmia  only.  It  has,  of  late 
years,  been  largely  supplanted  by  more  accurate  procedures. 


APPENDIX. 


127 


FIG.  74. — Showing  Placido's  Disc. 
OPHTHALMOMETER. 

The  Ophthalmometer  (Fig.  75)  is  a  device  used  for  the  de- 
termination of  corneal  astigmia.  It  consists  of  a  horizontal 
tube  or  telescope  mounted  upon  a  movable  tripod.  Within 
this  tube  is  a  combination  of  lenses  and  prisms  and  two 
reflectors,  technically  called  "mires,"  known  as  the  "steps" 
and  "parallelogram."  The  mires  are  illuminated  by  means 
of  electric  lamps. 

The  patient  is  placed  so  that  the  eye  to  be  examined  is  in 
proper  position  at  one  end  of  the  tube,  the  observer  focussing 


128 


REFRACTION. 


from  the  other  end.  When  the  instrument  is  properly  focussed, 
the  observer  will  see,  reflected  from  the  cornea  of  the  pa- 
tient's eye,  a  double  image  of  the  two  mires.  The  central 
image,  in  which  the  mires  are  close  together,  is  the  one  to 
be  observed. 

In  the  "primary"  or  first  position,  adjustment  is  made 
so  that  the  edges  of  the  mires  touch  and  the  black  lines  in 
each  are  continuous.  When  these  conditions  are  met,  the 
tube  is  rotated  ninety  degrees  to  the  right  or  left.  In  this 
way  any  difference  in  curvature  in  the  meridians  of  the  cor- 
nea will  be  indicated,  either  by  the  overlapping  or  separa- 
tion of  the  mires. 


FIG.  75. — Showing  Type  of  Improved  Ophthalmometer. 

The  mire  known  as  the  "  steps  "  is  made  up  of  eight  equal 
divisions.  The  amount  of  astigmia  is  measured  by  the  num- 
ber of  the  steps  overlapped  or  separated,  one  step  corre- 
sponding to  1  D.  of  astigmia.  The  amount  and  kind  of 
astigmia,  as  well  as  the  axis  of  the  correcting  cylinder,  are 
indicated  on  a  graduated  arc  attached  to  the  tube.  If  the 
image  remains  the  same  in  all  meridians  no  corneal  astigmia 
is  present. 

In  view  of  the  fact  that  lenticular  astigmia  is  not  meas- 
ured by  the  Ophthalmometer  the  results  obtained  do  not,  as 


APPENDIX.  129 

a  rule,  indicate  the  total  error  of  refraction.  The  instrument 
is  essentially  a  "  keratometer  "  and  its  use  constitutes  merely 
a  confirmatory  test. 

RISLEY'S  ROTATING  PRISM. 

This  instrument  (Fig.  76)  used  in  testing  insufficiencies  of 
the  external  ocular  muscles,  is  intended  to  do  away  with  the 
necessity  of  a  large  number  of  prisms.  It  consists  of  two  15° 
prisms  which  are  revolved  in  opposite  directions  by  a  milled 
head  screw,  thus  furnishing  a  prism  the  strength  of  which 
can  be  increased  from  0°  to  30°.  It  is  made  of  the  diameter 
of  trial  lenses  and  can  be  placed  before  the  eye  in  the  trial 
frame. 


FIG.  76. — Showing  Risley's  Rotating  Prism. 
PHOROMETER    (STEVENS'). 

The  Phorometer  is  an  instrument  designed  for  the  testing 
of  muscular  insufficiencies.  It  contains  two  cells  in  each  of 
which  rotates  a  5°  prism.  Each  cell  has  a  border  of  teeth,  a 
small  wheel  gearing  the  two  cells  together  and  causing  them 
to  rotate  in  opposite  directions,  thus  furnishing  a  prism  the 
strength  of  which  can  be  increased  from  0°  to  10°.  A  spirit 
level  is  placed  beneath  the  prisms  to  maintain  them  in  a 
horizontal  position. 

The  improved  instrument  (Fig.  77)  has  added  to  the 
above,  Risley's  Rotary  Prism  and  Maddox  Multiple  Rod  in 
such  manner  that  the  Phorometer  can  be  used  independently 


130 


KEFRACTION. 


of  the  other  two  or  in  conjunction  with  them.  It  is  equipped 
with  a  wall  bracket  swinging  freely  by  means  of  a  hinge 
connection  and  can  be  readily  raised  or  lowered  by  means  of 
a  geared  adjustment. 


FIG.  77. — Showing  Improved  Phorometer. 
PERIMETER. 

Of  no  valiie  in  obtaining  the  refraction,  this  instrument 
(Fig.  78)  is  used  to  measure  and  outline  the  visual  field  of  the 
eye.  It  consists  essentially  of  a  wide  metal  arc  fastened  to  a 
disc,  the  two  being  supported  by  an  upright.  A  small  black 


APPENDIX. 


131 


metal  plate  bearing  a  white  spot  is  clutched  to  the  arc  in 
such  a  manner  that  it  can  be  moved  freely  along  the  arc 
from  end  to  end.  The  arc  can  be  readily  rotated  to  lie  in  any 
meridian.  The  disc,  being  fastened  to  the  arc,  moves  with 
it,  its  purpose  being  to  hold  a  small  chart  on  which  the  pa- 
tient's visual  field  is  traced.  A  chin  rest  is  supplied  for  the 
purpose  of  steadying  the  patient's  head. 

The  patient,  having  been  placed  in  position,  is  instructed 
to  direct  his  gaze  toward  the  centre  of  the  arc  which  is  first 
placed  in  the  vertical  meridian.  The  small  metal  plate  bear- 


FiG.  78. — Showing  Perimeter. 

ing  the  \vhite  spot  is  then  moved  up  and  down  along  the  arc 
until  it  disappears  from  the  patient's  sight.  The  two  points 
on  the  arc  where  this  occurs  will  correspond  with  two  of 
the  degree  numbers  on  the  back  of  the  arc  and  are  indicated 
on  the  chart  by  means  of  dots  or  pin  pricks.  The  arc  is  then 
rotated  to  another  meridian  and  the  same  procedure  re- 
peated. When  all  meridians  have  been  tested  the  dots  on 
the  chart  are  joined  together  by  means  of  aline,  when  the 
field  will  have  been  outlined.  For  the  purpose  of  comparison, 
the  chart  bears  the  outline  of  a  normal  visual  field.  The  im- 


132  REFRACTION. 

proved  instrument  is  fitted  with  an  attachment  by  which 
the  results  are  automatically  recorded  on  the  chart.  The 
color  of  the  slide  may  be  changed  for  the  purpose  of  testing 
the  color  field. 

STIGM  ATOMETER . 

The  Stigmatometer  (Fig.  79)  is  an  instrument  for  testing 
the  refraction  of  the  eye  by  the  objective  method.  It  casts 
an  image  on  the  patient's  retina  which  is  plainly  visible  to 


FIG.  79. — Showing  Stimatometer. 

the  observer.  This  image  is  focussed  by  adjusting  the  instru- 
ment to  conform  to  the  refraction  of  the  eye  under  observa- 
tion, the  adjustment  being  read  off  on  the  scale  and  the  re- 
fractive error  indicated.  This  instrument  is  also  a  complete 
ophthalmoscope  for  the  direct  examination.  It  consists  of 
a  mirror,  a  lens,  a  screen,  light  and  an  operator's  lens  plate. 


APPENDIX.  133 

Use  as  an  Objective  Test. — The  patient  is  placed  with  his 
chin  on  the  rest  and  the  mirror  adjusted  before  the  eye 
to  be  examined,  the  light  being  reflected  into  the  pupil  so 
that  the  patient  sees  the  image  of  the  object.  With  the 
holder  containing  the  astigmic  dial  at  a  given  point,  the 
object  is  rapidly  moved  from  right  to  left  until  the  lines  (or 
part  of  them)  are  visible  to  the  operator.  This  image  is 
focussed  by  moving  the  screen,  the  figures  on  the  bar  indicat- 
ing in  diopters  the  amount  of  refractive  error.  If  in  this 
position  but  one  line  is  clearly  seen,  astigmia  is  present  and 
the  object  is  now  moved  back  until  the  line  at  right  angles 
is  distinct,  when  the  difference  in  the  two  readings  indicates 
the  cylinder  to  be  prescribed  in  addition  to  the  sphere  indi- 
cated. 

As  a  Subjective  Test. — In  using  the  stigmatometer  as  a 
subjective  test,  the  same  principles  apply  as  in  its  use  by  the 
objective  method,  the  difference  being  that  the  patient  is 
questioned  as  to  his  vision. 

By  withdrawing  the  screen,  which  is  readily  removable, 
the  instrument  becomes  an  ophthalmoscope. 


GENERAL  INDEX. 


Abbreviations, 26 

Abduction,  94 

Aberration,  chromatic, 102 

spherical, .  115 

Absolute  hyperopia, 58 

Accommodation, 37 

amplitude  of, 37 

diminution  of,  .  ...  43,  84 

in  hyperopia,        57 

in  myopia, 66 

in  presbyopia, 84,  86 

iris  in,         40 

mechanism  of, 37 

muscle  of,  ....  •    •    •    •    37 

near  point  of, 37 

paralysis  of, 60,  61 

physiology  of,  ...  •    -    •    37 

relationship  of,  to  age,  ....    86 

to  convergence,  .  40,  41,  57,  101 

to  pupillary  contraction,  .  40,  41 

relative  amplitude  of,   ...  40,  41 

spasm  of, 57 

Actual  deviation, 95,  96 

Acuity  of  vision, 26,  44 

Acuteness  of  vision,  ....  26,  44 

definition  of,        45 

in  astigmia, 71 

in  emmetropia 45 

in  hyperopia, 58 

in  myopia, 65 

method  of  recording, 45 

Adduction, 94 

Adjustable  eye  glasses, 112 

Advantages,  of  eye  glasses,  .   .    .  103 

full  frames, 103 

luminous  ophthalmoscope,  .      119 

retinoscope, 121 

periscopic  lens,      .      114,  115,  117 


rimless  or  skeleton  frame,  ..      103 

spectacles 103 

toric  lens,  ...  .    .  117 

Age, effect  of,  on  accommodation, 

40,  41,  43,  86 

convergence,  .        .  40,  41 

pupillary  contraction,   ....    41 

size  of  pupil,        43 

Aids  to  refraction, 118 

Amaurosis, 82,  83 

causes  of , 83 

Amblyopia, 82,  83,  95 

causes  of, 83 

Ametropia, 55,  89 

forms  of,        55 

Ametropometer, 123 

Amplitude  of  accommodation,  37,  40 

of  convergence, 39,  40 

Angle  of  convergence,  ....    39 

of  lenses, 107 

meter, 39 

prismatic, 17 

visual, 44 

Anisometropia, 82 

correction  of, 82 

Annular  muscle, 37 

Anomalies,  physical, 42 

Apex  of  prism,  ...        .    .  13,  97,  98 

Aphakia,   .  57,  82 

cause  of,  .    .  ....    82 

glasses  for, 82 

Appendix, 118 

Aqueductus  Sylvii, 40 

Aqueous  humor, 35 

Argand  burner, .88 

Associated  movement,  ....        95 
Assymetry  of  face  in  astigmia,    .    71 

in  hyperphoria,        101 

Asthenopia, 47,  96 


136 


GENERAL  INDEX. 


accommodative, 47 

as  a  cause  of  general  disease,  .  48 
convergence  or  muscular,  .  .  47 
symptoms  of, 47 

Astigmic  dial,  .  .  .  .  •  •  73 
as  seen  by  an  astigmic  eye,  .  74 
lens, 20 

Astigmia, 68 

according  to  rule,  .  .  69 

against  the  rule, 69 

acquired, 71 

assymetry  of  face  in, 71 

causes  of, 71 

compound  hyperopic,  .  .  .  70,  77 

myopic, 71,  78 

congenital, 71 

corneal, 69,  126,  127 

correction  of,  .........  74 

description  of, 68,  69 

determination  of, 71,  72 

fan  of  rays  in, 72 

forms  of 69,  70 

high  degree  of,  ...  92,  93 

irregnlar, 69 

latent, 73 

lenticular, 33,  71,  128 

low  degree  of, 92 

manifest, 73 

measurement  of, 75 

mixed, 71,  79 

movement  of  shadow  in,  .  .  .  93 
principal  meridians  of,  .  69,  70,  71 

regular, 69 

forms  of, 69 

simple  hyperopic, 69,  75 

myopic 70,  76 

symptoms  of, 71 

tests  for,  .  72,  73,  90,  91,  92,  93 
varieties  of, 69,  70 

Atropin, 60,  61 

Authors'  axis  finder, 31 

trial  frame, 52,  54 

Axial  hyperopia, 43 

myopia, 64 

Axis  finder 31,  32 

of  astigmia, 74 


cylinder, 20,  22 

eye, 39 

principal, 17 

B 

Balance,  muscular, 95 

Band  of  light, 93 

Base,  curve, 116,  117 

of  prism, 13,  102 

Beam  of  light, 15 

Bi-concave  lens, 24 

Bi-convex  lens, 24 

Bidwell,         41,  42 

Bifocal  lens, 113 

forms  of, 113 

Franklin, 113 

invisible, 114 

objections  to, 114 

Binocular  vision, 37,  95 

Blepharitis, 47 

Bridge, 104,  107 

crest  of, 108 

height  of, 105,  107,  108 

width  of,  105,  107 

Bulging  cornea, 64 

Burner,  Argand,  ... 

c 

Cataract  lenses, 82,  117 

Cement  bifocal, 113 

Centre,  optical, .  19 

Chimney,  re tinoscope,  .  .  .  88 

Choice  of  mydriatics, 60 

Chromatic  aberration, 102 

Ciliary,  body, 37 

muscles, 37 

in  the  hyperopic  eye,  ...  62 
nerve  supply  of,  ....  40,  100 

Circular  spring, 109 

Clerical  eye  glasses,' 112 

Cobalt  test, 99 

Cocain,  60,  61 

Compound  lens, 27 

Concave  lens 15,  16,  17 

mirror, 89 


GENERAL   INDEX. 


137 


Cone,  converging, 15 

diverging, 16 

Confirmatory  tests,  88, 89,  90, 91,92,93 

Conical  cornea, 64 

Conjunctival  irritation,     ....    47 

Contraction  of  pupil, 62 

Contraindications  for  use  of  myd- 

riatics, 60 

Convergence,  .  ....      37,  39 

amplitude  of,    ...  ...    39 

angle  of, 39 

asthenopia,  due  to, 47 

in  hyperopia, 57 

measure  of, 39 

near  point  of, 39 

relationship  of,  to  accommoda- 
tion,    ...          40,  41,  57,  101 
to  pupillary  contraction,  .  40,  41 
relative  amplitude  of,    ...        40 

Converging  cone, 15 

meniscus, 24,  115 

Convex  lenses, 15)  17 

Coquille, 24 

Cork  guards, no 

Cornea, 35 

astigmia  of, 69,  126,  127 

bulging  or  conical, 64 

Correction  of  results  under  myd- 

riatics, 61,  62 

Cover  test, 99 

Crest  of  bridge, 108 

Cross-eye, 95 

Crossed  cylinders, 79 

Crown  glass, n 

Crystalline  lens, 35,  36,  82 

hardening  of, 84 

Curve,  base, 116,  117 

Curvature,  hyperopia, 57 

myopia, 64 

Cycloplegics, 60,  61 

choice  of, 60 

contraindications  for,    ....    60 
correction  of  results  under,  61,  62 

duration  of  effect  of, 61 

strength  of, 61 

time  required  to  act .61 

10 


Cyclophoria, • 96 

determination  of, 100 

Cylinders,  crossed,     .....    79 

Cylindric  lens, 20 

axis  of, 20,  22 

displacement  by, 27 

effect  of, 21 

neutralization  of,  .  .    .    .  28,  29,  33 

D 

Data  for  ordering,  eye  glasses,   .  109 

spectacles, 105 

Decentering  a  lens,    ...        .        34 

prismatic  effect  of, 115 

Defects,  optical,  of  normal  eye,  41 
Deflection  of  light  rays,  .10,  u,  12 

Density,  optical, 10 

Deviation,  actual, 95,  96 

tendency  toward, 96 

Devices  for  measuring  lenses,  31,  32 

Dexter,      26 

Dial, 73,  74 

Dilatation  of  pupil, 60 

Displacement  by  lenses,  ....    27 

Diopter, 25 

Dioptric  system, 35 

Diplopia, 39,  95 

artificial, 97 

Direct  method, 89 

Disc,  optic, 90,  91 

pinhole 50,  51 

Placido's, 126,  127 

Displacement,  by  cylinders,  .    .    27 

by  prisms, 30,  97 

by  spheres,    ........    27 

by  sphero-cylinders, 27 

electrical, 9 

parallactic, 27 

Distance  of  lenses  from  the  eye,  108 

Diverging,  cone, 16 

meniscus, 24,  115 

squint, 66 

Double  prism,      52,  97 

Dropping  lenses,  ....  .    .  in 

Duboisin, 60,  62 

Duration  of  effects  of  mydriatics,  61 
Dynamic  refraction, 36 


138 


GENERAL   INDEX. 


E 

Elasticity  of  lens, 37,  43 

Electrical  displacement,  ....  9 
Electro-magnetic  radiations,  .  .  9 
Elongation  of  the  eyeball,  .  .  63,  65 

Emtuetropia, 35,  41 

Emmetropic  eye,    ....  35 

changes  in, 42,  43 

defects  of,  .  .  41 

life  history  of, 42,  43 

English  system, ..        25 

Ephedrin, 60,  61 

Eserin, 62 

Esophoria, 96 

cause  of, 101 

Ether,  luminiferous,      9 

vibrations  of,  10 

Euphthalmin, 60,  61 

Exophoria, 96 

associated  with,  myopia,     .    .     101 

External  rectus 94 

Extrinsic  muscles, 95 

Eye,  astigniic,  ...  .    .        69 

changes  during  life  history  of,  42 

defects  of,  .  .        41 

dioptric  system  of, 35 

emmetropic, 35 

hyperopic, 43,  56 

myopic, .    .  63,  64 

normal 35 

Eye-strain, 47 

Eye  glasses, 109 

adjustable, 112 

advantages  of, 103 

clerical,  112 

data  for  ordering,    .....  109 

half  oval,       in 

full  frame, 108,   109 

measurements  for, 109 

parts  of ,  .          109 

requisites  for  properly  fitting,  105 
rimless  or  skeleton,    ....'.  108 

F 

Face,  assy metry  of,  .    .  71,101 

Facultative  hyperopia,  .          .        58 


False  image,  ........        95 

Fan  of  rays,  .....    .....    72 

Far  point,  of  accommodation,         36 
of  convergence,  ......    39 

in  myopia,  .........        66 

Farsight,     ....  •  43,  55 

Faulty  meridian,  ...        73 

Field  of  fixation,  ........    95 

of  vision,    ..........  115 

First  principles,  ...     12 

Fixation,          ........  39,  94 

field  of,  .    .    .    ........    95 

Focal,  distance,   ........    23 

length,    ..........    23 

line,  ............    21 

Focal  lines,  position  of,  in  com- 

pound hyperopic  astigmia,  77 
myopic  astigmia,  ....  78 

mixed  astigmia,       ......    79 

simple  hyperopic  astigmia,  .    .    75 
myopic  astigmia,  ......    76 

Focus,.  ........    .    .    15 

negative,    ..........     16 

principal,   .......  15,  17,  22 

virtual,    ...........     16 

Fourth  ventrical,     .......    40 

Fovea  centralis,  ...  •    •    39 

Frames,  eye  glass,  .......  104 

spectacle,    ........  109 

Franklin  bifocal,  ........  113 

Functional,  disuse,     ......    95 

failure,    .........        84 

Fundus  reflex,        ......  89,  91 


Glass,  crown,    ......  n 

Glasses,  .....    .  ......  103 

requisites  for  properly  fitting,  105 
Glaucoma,  .........    60 

Guards,  ............  no 


II 


Half  oval  frames, 
Helniholz,       .    . 
Heterophoria,  .    . 
correction  of,  . 


.    .  Ill 

•  37,  JI9 

....    96 

.   IOO,   IOI,   IO2 


GENERAL   INDEX. 


139 


determination  of, 96,  97, 

forms  of, 96 

Heterotropia, .    .    95 

Hematropin, 60,  61 

Hook  fronts,  .        .    .    .    .  in 

Hoop  spring, 109 

Humor,  aqueous,     .    .  -35 

vitreous, 35,  36 

Hygiene  of  myopia,  .    .  65,  67 

Hyperesophoria, 96 

Hyperexoplioria,  96 

Hyperopia,  . 43,  55 

absolute, 58 

accommodation  in, 57 

axial, 43 

causes  of,  .  .    .  43,  57 

ciliary  muscle  in,    ...    .    .        62 

convergence  in,  .    .  •    •        57 

correction  of,  ......    58 

curvature,  .  ...  57 

description  of ,  .  .    .        .        56 

determination  of, 58 

facultative, 58 

latent,  ....        58 

manifest, 58 

near  point  in, 58 

relative,  .......  58 

sequelae  of,    .    . 57 

symptoms  of, 57 

total, 57 

varieties  of,  ....  ...    58 

Hyperopic  astigmia,    .  69,  70,  75,  77 

Hyperphoria, 96 

associated  with  assymetry,  .    .  101 
Hypophoria, 96 


Illiterates, 88 

Illusions,  optical, 42 

Image,  movement  of,    ....  90,  92 

Imbalance,  muscular,    .    .  94 

Inch  system,  25 

Index  of  refraction,    ...        .    .  n 

Indirect  method, 90 

Inferior  oblique, 94 

rectus, .  94 


Inset  bridge, 108 

Instruments  used  for  refraction,  118 

Internal  rectus,     ...        .  39,  40,  94 

nerve  supply  of,  ....  94 

Invisable  bifocal, .114 

Iiiterpupillary  distance,  ....  106 
Iris  in  accommodation,  .....  40 
Irregular  astigmia, 69 


K 


Keratoconus, 
Keratometer, 
Keratoscopy, 


64 
129 


Latent  astigmia, 73 

hyperopia, 58 

Latitude  of  motion,  .    .        95 

L/ens,  crystalline,     .    .          35,  36,  82 

hardening  of,      43,  84 

resiliency  of, 37 

striae  in,  . 42 

suspensory  ligament  of,    .    .        37 
Lenses,  abbreviations  for,    .        .    26 

action  of, 13,  23 

angle  of, 107 

astigmic, 20 

bi-concave, 24 

bi-convex,  ....        .        .    .        24 

bifocal, 113,  114 

cataract, 82,  117 

character  of, 27 

concave, 15,  16,  17 

convex,      15,  ^7 

compound,    -    .  ....    27 

coquille, 24 

cylindric, 20,  24 

effect  of, 21,  23,  28,  29 

measuring  strength  of ,  .    .        28 
properties  off    ...  .    .    23 

relationship  of,  .  .    .    .  32,  33,  34 

decentered, 34 

prismatic  effect  of, 115 

displacement  by, 27 

distance  from  the  eye  of,  105,  108 

dropping, .  in 

effect  of, 13,  23 


140 


GENERAL  INDEX. 


,     forms  of, 24,  27 

measuring,  .  .  .    .      28,  29,  30 

mechanical  devices  for,      31,32 

meniscus,  converging,    .    .  24,  115 

diverging, 24,  115 

minus, -        .        16 

neutralization  of, 26,  27 

numeration  of, 26,  49,  50 

periscopic, 114 

plane  of, 105 

piano, 24 

plano-concave,  ....  .    24 

-convex, 24 

-cylindric,      20 

plus,  15 

prescription  for,  ......    26 

prismatic, 13,  24 

effect  of, 29,  34 

measuring  strength  of,  ...    29 

properties  of, 29 

relationship  of,  .       .    .  32,  33,  34 

sizes  of,      107 

special  forms  of, 113 

spheric, 14,  15 

effect  of, 14,  23 

measuring  strength  of,  ...    28 

properties  of,  23 

relationship  of,     .    .    .32,  33,  34 

sphero-cylindric, 24 

strength  of,   ...  ...    27 

surfaces  of, 17,  23 

systems  of  measurement  of,  24,  25 

toric, .    .  24,  115,  116 

varieties  of, 23,  24 

Lenticular  astigmia,  .  .    .  33,  71,  128 

Ligament,  suspensory, 37 

Light,  band  of, 93 

deflection  of, 10,  n,  12 

ray 9 

sensation  of, .    .      9 

wave  theory  of,    .        9 

Line,  focal, •    .    21 

Loring  ophthalmoscope,  .    .    .    .118 

Luminiferous  ether, 9 

Luminous  ophthalmoscope,     .    .119 
retinoscope 119 


Macula  lutea,      94,  95 

Maddox  rod, 51 

test, 97,  98 

Malingering, 83 

Manifest  astigmia 73 

hyperopia,  58 

Measurements   for  ordering,  eye 

glasses,  .    .        .    .      109 

spectacles, 105 

Measuring  lenses,    ....  28,  29,  30 

Mechanical,  aids  to  refraction,  .  118 

devices  for  measuring  lenses,  3 1 ,  32 

Mechanism  of  accommodation,  .    37 

Medium,  refracting, 10 

transmitting, 10 

Meniscus, 115 

converging, 24,  115 

diverging,       ....        .   .  24,  115 

Meridian,  faulty,     .......    73 

normal, 73 

principal, 115,  116 

Metal  guards, no 

Meter, 25 

angle,  ....  39 

lens, 25 

Metric  system,  ...  .    .        25 

•  Minus  lens,  ....  .    .  16 

Mires, 127,  128 

Mirror,  concave, 92 

piano, 92 

retinoscopic, 89,  92 

test 89 

Mixed  astigmia, 71,  79 

Monoyer, 25 

Motor  oculi, 40 

Movements,  associated,    ....    95 

ocular, 94 

Miiller,  annular  muscle  of,  .    .        37 

Muscles,  annular, 37 

balance  of, 95 

ciliary, 37 

extrinsic, 94 

imbalance  of, 94 

ocular 94 

moving  the  eyeball,  ....    94 


GENERAL  INDEX. 


141 


of  accommodation, 37 

tests  of,  ....  96,  97,  98,  99,  100 

Mydriatics,       ...  .    .    .  60,  61 

choice  of,  .  .    .  .    .    60 

contraindications  for  use  of,  .  60 
correction  of  results  under,  61,  62 

duration  of  effect  of, 61 

strength  of, 61 

time  required  to  act, 61 

Mydrin 60,  61 

Myopia, 63 

accommodation  in, 66 

axial,  .  64 

causes  of,  ....  65 

correction  of, 66 

curvature, ...  64 

description  of,  : 63,  64 

determination  of,  .  .  .  .  66 

forms  of, 64 

hygiene  of,  ....  .    .  65,  67 

measurement  of,  .  ....    66 

ophthalmoscopic  appearance  in,  66 
progressive,  .    .  ....    65 

prophylaxis,  ...  .  67 

symptoms  of ..........    65 

treatment  of,     ...  .    .    67 

varieties  of, 64 

Myotics, 62 


Nagel, 25 

Near  point,  of  accommodation,  .    37 

of  convergence, 39 

in  hyperopia, 58 

recession  of,    ......  84,  85 

sight,      63 

Negative  focus, 16 

Nerve,  motor  oculi, 40 

Neutralizing  lenses,      .        .  26 

Normal  eye, 35 

changes  in,       ...  42,  43 

defects  of, 41 

life  history  of,     ....          42,  43 

Nose  piece, 104,  107,  108 

Numeration  of  lenses,   .    .  26,  49,  50 


O 

Objective,  lens 89 

tests,  .    .       88,  89,  90,  91,  92,  93 
Oblique,  inferior,  .    .  94 

superior, 94 

Oblong  spring, 109 

Obturator,  .       .  ....        50 

Ocular,  movements, 94,  95 

muscles,  .  .    .        94 

Oculi  utrique, 26 

Oculus  dexter,     .    .  ...        26 

sinister, 26 

Operation  for  strabismus,  ....  102 

Ophthalmometer, 127,  128 

Ohthalmometroscope,  .    .      122,  123 

Ophthalmoscope,    .•  .  118,  119 

Loring's,    ....  .    .    .    .  118 

luminous, 119 

Ophthalinoscopy , 89,  90 

Optic  disc,  89,  90 

Optical,  centre,         .        .    .      19,  105 

defects, 41 

density,      .    .  .    .    .    .     10 

illusions, 42 

principles, 9,  12 

Orthophoria, 95 

Oscillations  of  ether,  ...  10 

direction  of, 10 

rate  of,  10 

uniformity  of, 10 

Outset  bridge, ...  108 

Offset  stud,  ...        no 


Parallactic  displacement,  ...        27 

Parallelogram, 127,  128 

Paralysis  of  accommodation,    60,  61 

Perimeter, 130,  131 

Periscopic  lens,   ...        .  114 

Phorometer,  .          129 

Physical  anomalies,        .    .        .42 
Physiology  of  accommodation,       37 

Pilocarpin, 62 

Pin  hole  disc, 50,  51 

Placido's  disc, 126,  127 

Plane  of  lenses, 105 


142 


GENERAL   INDEX. 


Piano, 24 

concave, 24 

convex, ...        24 

cylindric, 20 

Plus  lens, 15 

Position,  primary,  .    .        .    .          128 

Presbyopia, 43,  84 

age  of  onset, 84 

causes  of, 43,  84 

correction  of, .86 

description  of , 43,84,  85 

glasses  for, 86,  87 

relationship  of,  to  age,  .        .        86 
recession  of  near  point  in,     84,  85 

symptoms  of, 84 

Prescription  writing,  ...  26 

Primary  position, 128 

Principal,  axis, 17 

foCUS,    .     .  15,    17,    22 

Principles,  optical, 12 

Prism,  apex  of,    ...          13,  97,  98 

base  of, 13,  102 

correcting, 98,  102 

deflection  by,  ....       14,  29,  97 
displacement  by,  ...          30,  97 

double, 52,  97 

effect  of, 29,  30 

exercise, 101 

neutralization  of,     .  .    .    30 

numeration  of, 50 

position  of  correcting,     .      98,  102 

Risley's, 129 

sides    forming    spherical    sur- 
faces, 17,  18. 

uses  of, 97,  98 

wearing  of, 101 

Prismatic,  angle,     ....  17 

effect  of  decentering,  .         34,  115 

Properties  of  lenses, 23 

Prophylaxis  of  myopia,     .    .  67 

Punctum  proximum,  ...    .    .        37 

of  accommodation, 37 

of  convergence, 39 

in  hyperopia, 58 

recession  of, 84,  85 

Punctum  remotum, 36 


of  accommodation, 36 

•of  convergence,  ....  39 

in  myopia,  66 

Pupil,  activity  of, 43 

con  traction  of,  ...  40,  61,  62 

dilatation  of, 60 

relationship  of  size  to  age,  43 

to  accommodation, 40 

to  convergence,  .  .  .  .  40 
sphincterof, 40 

Pupillary  distance, 106 

line,  ....  .  .  .  .  108 

R 

Radiations,  electro-magnetic,  .  9 

light,  ...  9 

Range,  of  accommodation,  .  .  37 
of  convergence,  .  -39 

Rays, 9 

convergent, 15,  23 

course  of,  12 

deflection  of,  .  12 

divergent, 23 

fan  of, 72 

incident, .  10 

parallel, 23 

refracted,  10 

velocity  of,  .  .  .  .  12 

Recession  of  near  point,  .  .      84,  85 

Rectus,  externus, 94 

inferior, 94 

internus, 39,  40,  94 

nerve  supply  of,  roo 

superior,  .  .  ...  94 

Reflex,  fundus,  ....  .  .  89 

brightness  of, 91 

movement  of,  ....  .  .  91 

Refraction, ......  TO 

aids  to, .  .  118 

by  cylinders, 21 

by  prisms, 14,  29,  97 

by  spheres, 15,  16 

defined n 

dynamic, ...  36 

index  of,  ...  .  .  .  .  n 

laws  of, 12 

static,  .  .  .......  36 


GENERAL  INDEX. 


143 


Regular  astigniia,    .......    69 

Relationship  of  lenses,  .  .  32,  33,  34 
Remotum,  punctum,         ....    36 

Requisites    of,    properly    fitting 

glasses,    .  105 

vision, 35 

Retinal  image,  in  astigmia,     .        90 

in  hyperopia, 90 

in  myopia, 90 

Retinoscope, 89 

luminous, 120,  121 

Retinoscopy,     .    .        91 

chimney,        .    .    88 

Reversible  spectacles, in 

Risley's  prism, 129 

S 

Scopolamin, 60,  61 

Second  sight, 65 

Sequelae  of  hyperopia, 57 

Shadow,  movement  of,    .    .      91,  92 

shape  of, 91,  92 

size  of, 91,  92 

test, 91 

Shell  guards, no 

Simple,  hyperopic  astigmia,    69,  75 
myopic  astigmia,    .        ...  70,  76 

Sinister, .26 

Sizes  of  lenses, 107 

Skiascopy, '91 

Snellen  test  letters,        44 

principle  involved  in,    ....    44 
Spasm  of  accommodation,    ...    57 

symptoms  of, 58 

treatment  of, 59 

Spectacles, 103 

advantages  of, 103 

full  frame, 103 

measurements  for, 105 

requisites  for  properly  fitting,  105 

rimless  or  skeleton, 103 

Spherical,  aberration,     .        ...  115 

lens, 14,  17,  27 

Sphincter,  of  ciliary  body,  .    .    -37 

of  pupil, 40 

Split  bi-focal, 113 


Springs, 109 

Squint, 95 

divergent, 66 

Standard  lens, 25 

sizes, 107 

Staphyloma, 65 

Static  refraction, 36 

Stenopseic  disc, 50,  80 

Steps, 127,  128 

Stevens'  phorometer, 129 

Strabismus,      •.  •  57,  95 

divergent, 66 

Streak  of  light 98,  99 

Striae  of  lens, 42 

Studs, ,   .    no 

Superior,  oblique, 94 

rectus, 94 

Surface,  refraction  at, n 

Sursumduction, .    .    94 

Suspensory  ligament, 37 

Sylvii,  aqueductus, 40 

Symptoms  of,  asthenopia,    ...    47 

astigmia, 71 

hyperopia, •    •    •    •    57 

myopia, 65 

presbyopia, 84 

System,  dioptric, 35 

English, 25 

metric,    ...        25 

T 

Temples, 104,  108 

forms  of,    .    .' 105,  108 

length  of, 108 

Test,  card, 45,  46 

frame, 52 

letters, 44 

mirror,     .  89 

shadow, 91 

Tests,  confirmatory, 

88,  89,  90,  91,  92,  93 

cover, .        .    .    99 

for  astigmia, 71-80 

cyclophoria, 100 

field  of  vision, 130,  131 

hyperopia, 58 


144 


GENERAL  INDEX. 


malingering, 83 

muscles, 96,  97,  98,  99 

myopia, 66 

Theory  of  light, 9 

Third  nerve, 100 

Thorington  chimney, 88 

Total  hyperopia, 57 

Tor, 115 

Toric  lens, 115,  116 

advantages  of , 117 

Trial,  case, 49 

frame, 52 

True  image, 95 

U 
Utrique, 26 


Varieties,  of  astigmia,  ....  69,  70 

bi-focals, 113,  114 

heterophoria, 96 

hyperopia, 58 

lenses, 23,  24 

myopia, 64 


Ventricle,  fourth,        40 

Vertical  springs 109 

Vibrations  of  ether, 10 

Virtual  focus, 16 

Vision,  acuteness  of, 26 

binocular, 37,  95 

field  of ....  115 

requisites  of, 35 

Visual,  acuity, 26,  44 

measure  of, 45 

angle,      44 

field, 115 

Vitreous  humor, 35 ,  36 

W 

Wafer  lens, .  113,  114 

Wave  theory  of  light, 9 

Wedge-shaped,  beam,  .    .    .    .  21,  68 
lens 13 


Young,   .        33 


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